sourcemaps upload: require release or debug ids

[loewenheim]

Sentry can't do anything with uploaded files unless a release is specified or they have debug ids. Therefore, sentry-cli should behave as follows:

1. If a release is given, all is well.
2. If no release is given and all relevant files have debug ids, all is well.
3. If no release is given and no relevant files have debug ids, this is an error and the upload should fail (the resulting bundle would be unusable).
4. If no release is given and some relevant files don't have debug ids (but some do), a warning should be printed, but the upload should still succeed (the resulting bundle is at least partly usable, but there is likely some misconfiguration or user error).

By "relevant file" I mean a minified source file that refers to a sourcemap, i.e. a file that could benefit from having a debug id.