ls: Give non-zero exit code when an unknown resource is specified

[kyleam]

This allows a command-line caller to use reproman ls to check if a resource exists, as described at gh-521.

[codecov[bot]]

Codecov Report

Merging #531 into master will decrease coverage by 0.09%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #531      +/-   ##
==========================================
- Coverage   89.66%   89.56%   -0.10%     
==========================================
  Files         148      148              
  Lines       12400    12432      +32     
==========================================
+ Hits        11118    11135      +17     
- Misses       1282     1297      +15     

Impacted Files	Coverage Δ
reproman/interface/ls.py	94.64% <100.00%> (+1.02%)	:arrow_up:
reproman/interface/tests/test_ls.py	100.00% <100.00%> (ø)
reproman/distributions/tests/test_venv.py	87.50% <0.00%> (-10.58%)	:arrow_down:
reproman/tests/skip.py	93.25% <0.00%> (-4.50%)	:arrow_down:
...eproman/interface/tests/test_backend_parameters.py	95.65% <0.00%> (-4.35%)	:arrow_down:
reproman/interface/run.py	100.00% <0.00%> (ø)
reproman/support/jobs/tests/test_orchestrators.py	94.98% <0.00%> (+0.09%)	:arrow_up:
reproman/support/jobs/orchestrators.py	93.58% <0.00%> (+0.32%)	:arrow_up:

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[kyleam]

Also pr description just says "as described at gh-521" - isn't it also closing it?

The second commit has a "Closes ..." statement.

[yarikoptic]

Great, thank you. Let's merge when turns green