Don't include errored cmd output twice.

[lkysow]

Since we're including the command output inside the error, we don't need
to also return the output. This was resulting in the error output being
printed twice.

Fixes #519

[codecov]

Codecov Report

Merging #619 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #619      +/-   ##
==========================================
- Coverage   72.17%   72.15%   -0.03%     
==========================================
  Files          61       61              
  Lines        4633     4629       -4     
==========================================
- Hits         3344     3340       -4     
  Misses       1044     1044              
  Partials      245      245

Impacted Files	Coverage Δ
server/events/runtime/run_step_runner.go	100% <100%> (ø)	⬆️
server/events/models/models.go	73.98% <0%> (-0.62%)	⬇️
server/events/working_dir.go	74.69% <0%> (-0.31%)	⬇️

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