[grouping] Handle trying to save bad rules more gracefully

- Investigate what actually comes back on the FE when you try to save bad rules. We could probably handle this better/give the user better information about what's wrong.
- Right now trying to save bad rules causes an error in Sentry, which probably isn't necessary.

Example sentry error (key error when trying to parse the bad enhancement rule, because the matcher isn't one we recognize): https://sentry.sentry.io/issues/7395241875/events/564314b44c5049b69762dadb2bc56307/

Start by reproducing the attempt to save an invalid enhancement rule and inspect what the frontend receives, using the linked Sentry event as context for the unrecognized matcher error. Done means invalid rules produce useful information for the user without generating an unnecessary Sentry error.

Written by the indexing model from the issue text.