Better error message when try_to_run command fails

[takluyver]

The CalledProcessError object should always have a .output attribute, although it may be None, so the getattr() will never hit the fallback condition.

The logical or operator should use the fallback if e.output is either None or an empty string, because those are both false-y. Any other possible value will be shown.

[codecov]

Codecov Report

Merging #201 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #201   +/-   ##
=====================================
  Coverage       0%     0%           
=====================================
  Files           1      1           
  Lines           1      1           
=====================================
  Misses          1      1