Write error comment on early failures.

[lkysow]

Previously, we were just logging when an error happened early in our
comment processing code. However it's possible for us to write comments
on error even at this early stage which will make for a better user
experience.

Fixes #398

[codecov]

Codecov Report

Merging #401 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
- Coverage   69.88%   69.84%   -0.04%     
==========================================
  Files          62       62              
  Lines        3815     3817       +2     
==========================================
  Hits         2666     2666              
- Misses        960      961       +1     
- Partials      189      190       +1

Impacted Files	Coverage Δ
server/events/command_runner.go	71.32% <0%> (-1.07%)	⬇️

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