Improve alerting when sourcemap or debug file uploading fails

[szokeasaurusrex]

When sourcemap or debug file uploading fails, the Sentry CLI exits with a nonzero exit code. However, Sentry CLI users (including plugins that use Sentry CLI) may choose to ignore this code to prevent the Sentry CLI from failing builds, and may instead only log a warning, which end users may ignore. As a result, end users may fail to realize that the uploading process failed, and as a result, they could get events in Sentry missing symbolication.

We should try to improve how we alert users when sourcemap/debug file uploading fails. We should investigate whether it might be possible to alert users directly through Sentry. Likely, this will require creating a new issue type or some other new mechanism for alerting users.