Change log level for missing file case

[djaglowski]

When a file cannot be opened by the file_input
operator, we currently print an error log. In cases
where the same file is repeatedly unopenable, this will
spam error logs every poll interval. By making this a
debug log, users can still discover the issue if closely
concerned.

Resolves #342

[codecov]

Codecov Report

Merging #357 (96b7e43) into main (ad3817c) will decrease coverage by 0.0%.
The diff coverage is 0.0%.

@@           Coverage Diff           @@
##            main    #357     +/-   ##
=======================================
- Coverage   77.2%   77.1%   -0.1%     
=======================================
  Files         94      94             
  Lines       4471    4471             
=======================================
- Hits        3452    3449      -3     
- Misses       699     702      +3     
  Partials     320     320             

Impacted Files	Coverage Δ
operator/builtin/input/file/file.go	72.3% <0.0%> (ø)
...perator/builtin/transformer/recombine/recombine.go	74.3% <0.0%> (-2.1%)	⬇️