Allow users to cancel running jobs

[kofalt]

Also added an exception change, so that heartbeating a canceled job doesn't throw an exception.

Review Checklist

• Tests were added to cover all code changes
• Documentation was added / updated
• Code and tests follow standards in CONTRIBUTING.md

[nagem]

LGTM, feel free to merge when CI is green.

[codecov-io]

Codecov Report

Merging #1032 into master will not change coverage. The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #1032   +/-   ##
=======================================
  Coverage   90.61%   90.61%           
=======================================
  Files          50       50           
  Lines        6766     6766           
=======================================
  Hits         6131     6131           
  Misses        635      635