Improved error logging

[AnHeuermann]

Fixes #67.

Changes

• New error objects for general omc errors and timeout errors.
  • Show full messages from stdout and stderr to user if omc failes to start.
• Use == and ==! when comparing to nothing
• Fix Project.toml for older Julia 1.0/ 1.1
• Update version to 0.2.1

julia> omc=OMJulia.OMCSession()
[ Info: Path to zmq file="/tmp/openmodelica.USER.port.julia.oNnqddn7qq"
ERROR: OMCError
Command `omc --interactive=zmq +z=julia.oNnqddn7qq` failed
Error: You are trying to run OpenModelica as a server using the root user.
This is a very bad idea:
* The socket interface does not authenticate the user.
* OpenModelica allows execution of arbitrary commands.

Execution failed!

Stacktrace:
 [1] OMJulia.OMCSession(omc::Nothing)
   @ OMJulia /workspaces/OMJulia.jl/src/OMJulia.jl:174
 [2] OMJulia.OMCSession()
   @ OMJulia /workspaces/OMJulia.jl/src/OMJulia.jl:89
 [3] top-level scope
   @ REPL[2]:1

julia> omc=OMJulia.OMCSession()
[ Info: Path to zmq file="/tmp/openmodelica.USER.port.julia.jWW4HDd8CY"
ERROR: TimeoutError
ZMQ server port file "/tmp/openmodelica.USER.port.julia.jWW4HDd8CY" not created yet.
Stacktrace:
 [1] OMJulia.OMCSession(omc::Nothing)
   @ OMJulia ~/workdir/julia/OMJulia.jl/src/OMJulia.jl:179
 [2] OMJulia.OMCSession()
   @ OMJulia ~/workdir/julia/OMJulia.jl/src/OMJulia.jl:89
 [3] top-level scope
   @ REPL[2]:1

[AnHeuermann]

The used Julia is so old, that pipeline can't handle IOBuffer, so I need to use text files.