fix(core): Avoid showing success message if upload was disabled or nothing was uploaded

[Lms24]

This bugged me and many users for a while: Because of a weird control flow in the debugID source maps upload plugin, we'd print the "Successfully uploaded source maps to Sentry" message if

• a bunch of important preconditions for uploading were not met (e.g. missing auth token).
  • I think this was actually a pretty bad bug because it would cause Sentry CLI to do validation when it was called although we could have avoided this entirely.
• we didn't upload anything (i.e. we found 0 assets)

This PR fixes both cases.

closes #670

[chargome]

🥇