Improve error messages when bundling sources

[szokeasaurusrex]

When we are unable to identify the correct sources to bundle when uploading debug files, we should output detailed error messages to help users understand:

• Why we cannot find the source files, and
• What they can do to fix the problem'

This will help avoid confusion such as in https://github.com/getsentry/sentry-cli/issues/2186#issuecomment-2532090116