Extra info from command line in exception comments

Builds on !428 (merged) , adds an optional "--exc-comment" command line arg to improve usability of the comments placed on MRs where we hit an unexpected exception. If you're running in CI, you can pass something like --exc-comment "See log $CI_JOB_URL" and those comments will link to the log that has the full exception info (from !428 (merged))

Additionally, the bot was posting "i'm broken inside" as much or more than actually merging things, confusing other devs on the project. So I have changed it to a more emotionally-neutral (if boring) message.