keep kaldi-serve from crashing if an expected model is not found

[tanmay17061]

With this change, kaldi serve is expected to not crash due to model not found at specified path during ChainModel creation.

Old behaviour:

1. If a model is not found, KALDI_ERR macro is used to log error.
2. KALDI_ERR will print to STDERR and raise a std::runtime_error.
3. This uncaught std::runtime_error will lead to program exit.

New behaviour:

1. If a model is not found, KALDI_ERR macro is used to log error.
2. KALDI_ERR will print to STDERR and raise a std::runtime_error.
3. This std::runtime_error is caught, and entry for the model is not created in the DecoderQueue map.

[tanmay17061]

Testing setup:
Config:

[[model]]
name = "english-v5-new-structure"
language_code = "en"
path = "/home/app/models/english-v5-new-structure"

[[model]]
name = "somemodel"
language_code = "en"
path = "/home/app/models/somemodel"

with /home/app/models/english-v5-new-structure existent, but /home/app/models/somemodel missing.

Output:

:: Loading 2 models
::   - english-v5-new-structure (en)
::   - somemodel (en)
:: Loading model from /home/app/models/english-v5-new-structure
LOG ([5.5.0~1-da93]:RemoveOrphanNodes():nnet-nnet.cc:948) Removed 1 orphan nodes.
LOG ([5.5.0~1-da93]:RemoveOrphanComponents():nnet-nnet.cc:847) Removing 2 orphan components.
LOG ([5.5.0~1-da93]:Collapse():nnet-utils.cc:1472) Added 1 components, removed 2
WARNING ([5.5.0~1-da93]:ChainModel():model/model-chain.cpp:52) Word boundary file/home/app/models/english-v5-new-structure/word_boundary.int not found. Disabling word level features.
WARNING ([5.5.0~1-da93]:ChainModel():model/model-chain.cpp:77) RNNLM artefacts not found. Disabling RNNLM rescoring feature.
LOG ([5.5.0~1-da93]:ComputeDerivedVars():ivector-extractor.cc:183) Computing derived variables for iVector extractor
LOG ([5.5.0~1-da93]:ComputeDerivedVars():ivector-extractor.cc:204) Done.
LOG ([5.5.0~1-da93]:CompileLooped():nnet-compile-looped.cc:345) Spent 0.137849 seconds in looped compilation.
:: Loading model from /home/app/models/somemodel
ERROR ([5.5.0~1-da93]:Input():kaldi-io.cc:756) Error opening input stream /home/app/models/somemodel/HCLG.fst

[ Stack-Trace: ]
/opt/kaldi/src/lib/libkaldi-base.so(kaldi::MessageLogger::LogMessage() const+0xa71) [0x7fe85b26b50f]
/opt/kaldi/src/lib/libkaldi-decoder.so(kaldi::MessageLogger::LogAndThrow::operator=(kaldi::MessageLogger const&)+0x11) [0x7fe85ce63e13]
/opt/kaldi/src/lib/libkaldi-util.so(kaldi::Input::Input(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool*)+0x90) [0x7fe85b972b9c]
/opt/kaldi/src/lib/libkaldi-fstext.so(fst::ReadFstKaldiGeneric(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool)+0x3d) [0x7fe85c7734b5]
/usr/local/lib/libkaldiserve.so(kaldiserve::ChainModel::ChainModel(kaldiserve::ModelSpec const&)+0xb0f) [0x7fe85ea5710f]
/usr/local/lib/libkaldiserve.so(kaldiserve::DecoderFactory::DecoderFactory(kaldiserve::ModelSpec const&)+0x140) [0x7fe85ea53f30]
/usr/local/lib/libkaldiserve.so(kaldiserve::DecoderQueue::DecoderQueue(kaldiserve::ModelSpec const&)+0x237) [0x7fe85ea54b67]
./kaldi_serve_app(KaldiServeImpl::KaldiServeImpl(std::vector<kaldiserve::ModelSpec, std::allocator<kaldiserve::ModelSpec> > const&)+0xe1) [0x565374835c11]
./kaldi_serve_app(run_server(std::vector<kaldiserve::ModelSpec, std::allocator<kaldiserve::ModelSpec> > const&)+0x22) [0x56537483a4d2]
./kaldi_serve_app(main+0x661) [0x565374808851]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5) [0x7fe85e3b3b45]
./kaldi_serve_app(_start+0x2a) [0x5653748092ea]

ERROR ([5.5.0~1-da93]:ChainModel():model/model-chain.cpp:110) kaldi::KaldiFatalError

[ Stack-Trace: ]
/opt/kaldi/src/lib/libkaldi-base.so(kaldi::MessageLogger::LogMessage() const+0xa71) [0x7fe85b26b50f]
/usr/local/lib/libkaldiserve.so(+0xc7f4a) [0x7fe85ea1af4a]
/usr/local/lib/libkaldiserve.so(kaldiserve::ChainModel::ChainModel(kaldiserve::ModelSpec const&)+0x1e20) [0x7fe85ea58420]
/usr/local/lib/libkaldiserve.so(kaldiserve::DecoderFactory::DecoderFactory(kaldiserve::ModelSpec const&)+0x140) [0x7fe85ea53f30]
/usr/local/lib/libkaldiserve.so(kaldiserve::DecoderQueue::DecoderQueue(kaldiserve::ModelSpec const&)+0x237) [0x7fe85ea54b67]
./kaldi_serve_app(KaldiServeImpl::KaldiServeImpl(std::vector<kaldiserve::ModelSpec, std::allocator<kaldiserve::ModelSpec> > const&)+0xe1) [0x565374835c11]
./kaldi_serve_app(run_server(std::vector<kaldiserve::ModelSpec, std::allocator<kaldiserve::ModelSpec> > const&)+0x22) [0x56537483a4d2]
./kaldi_serve_app(main+0x661) [0x565374808851]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5) [0x7fe85e3b3b45]
./kaldi_serve_app(_start+0x2a) [0x5653748092ea]

Model not loaded: (somemodel,en). This raised an error:kaldi::KaldiFatalError
kaldi-serve gRPC Streaming Server listening on 0.0.0.0:5016

ListModels RPC responds with:

models:{name:"english-v5-new-structure"  language_code:"en"}

that is, the missing model is not present in the RPC response.

Calling Recognize with the missing model results in response:

error: <_InactiveRpcError of RPC that terminated with:
        status = StatusCode.UNKNOWN
        details = "[random] (KaldiServe Client) Recognize recognition config validation failed 
        ==>> ValidationError 
        ==>> (ERROR) model (somemodel) not supported by Kaldi. (recognition_config.model)"
        debug_error_string = "UNKNOWN:Error received from peer ipv6:%5B::1%5D:5018 {grpc_message:"[random] (KaldiServe Client) Recognize recognition config validation failed \n\t==>> ValidationError \n\t==>> (ERROR) model (somemodel) not supported by Kaldi. (recognition_config.model)", grpc_status:2, created_time:"2023-05-23T15:12:14.014718+05:30"}"
>