ENH: small tune ups to "jobs -s" output in case of missing external (datalad)

[yarikoptic]

See individual commits. Now (original version) would show

$> reproman --dbg jobs -s
[status: N/A - needs datalad] 20200107-213600-f814 on smaug via condor$ '{inputs}' '{outputs}' participant --participan...
...

instead of immediately crashing with

$> reproman  jobs -s    
2020-05-25 20:18:57,108 [ERROR  ] DataLad is required for orchestrator 'datalad-pair-run' is missing. [orchestrators.py:__init__:488] (MissingExternalDependency) 

edit1: pushed fb4e2a680a37c1fe660948263d554df1019ea109 which would just catch all exceptions and issue warning while providing NA for the status

[codecov[bot]]

Codecov Report

Merging #516 into master will increase coverage by 0.11%. The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #516      +/-   ##
==========================================
+ Coverage   89.47%   89.59%   +0.11%     
==========================================
  Files         148      148              
  Lines       12290    12295       +5     
==========================================
+ Hits        10997    11016      +19     
+ Misses       1293     1279      -14     

Impacted Files	Coverage Δ
reproman/support/jobs/orchestrators.py	91.84% <ø> (ø)
reproman/interface/jobs.py	97.14% <77.77%> (-1.86%)	:arrow_down:
...eproman/interface/tests/test_backend_parameters.py	100.00% <0.00%> (+4.34%)	:arrow_up:
reproman/tests/skip.py	97.75% <0.00%> (+4.49%)	:arrow_up:
reproman/distributions/tests/test_venv.py	98.07% <0.00%> (+10.57%)	:arrow_up:

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