Simplify error logs

[Mr0grog]

Right now, we log a complete stack trace for errors to Slack. For example: https://sfbrigade.slack.com/archives/C01AV92EM47/p1615716833000100

That's nice and detailed, but it's very hard to sort through and keep track of, especially when there is more than one error (hard to see where one stops and the next begins).

Some possible improvements here:

• Only log the error message (and maybe the location of the final/deepest stack frame). To support debugging, add a --verbose or --debug CLI option to log the full stack trace.

• Do the above for known/expected error types (e.g. FormatError) but not other types.

• Implement the Slack logger in Python here (what we originally did) instead of in the GitHub workflow. That allows us to do nice formatting so the errors are more readable.

• Some combination of the above.