ENH: prefix with [DATALAD] if save came without msg

[yarikoptic]

up for discussion

[mih]

+1, but please merge this after the current PR if it leads to conflicts.

[coveralls]

Coverage decreased (-2.08%) to 86.803% when pulling 3efbee1 on yarikoptic:enh-save into 014a507 on datalad:master.

[codecov-io]

Codecov Report

Merging #768 into master will decrease coverage by 2.07%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #768      +/-   ##
=========================================
- Coverage   88.88%   86.8%   -2.08%     
=========================================
  Files         214     214              
  Lines       18843   18845       +2     
=========================================
- Hits        16748   16358     -390     
- Misses       2095    2487     +392

Impacted Files	Coverage Δ
datalad/interface/save.py	91.89% <100%> (+0.46%)	⬆️
datalad/crawler/nodes/s3.py	32.82% <0%> (-57.26%)	⬇️
datalad/tests/test_s3.py	43.47% <0%> (-56.53%)	⬇️
datalad/downloaders/s3.py	41.73% <0%> (-51.31%)	⬇️
datalad/crawler/nodes/tests/test_s3.py	49.59% <0%> (-50.41%)	⬇️
datalad/customremotes/tests/test_datalad.py	59.25% <0%> (-40.75%)	⬇️
datalad/downloaders/tests/test_s3.py	67.92% <0%> (-28.31%)	⬇️
datalad/interface/ls.py	52.56% <0%> (-21.74%)	⬇️
datalad/support/s3.py	27.09% <0%> (-20.2%)	⬇️
datalad/downloaders/tests/test_http.py	81.74% <0%> (-14.07%)	⬇️
... and 6 more

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