Gazelle should exit with an error code when encountering errors in a development repository

[jayconrod]

First raised on bazel-go-discuss by @hochhaus.

Gazelle is fairly error-tolerant because we use it in go_repository, and we need it to partially succeed when it runs into invalid code (this is especially a problem in testdata directories for tools that operate on Go code).

It doesn't make sense for Gazelle to be so tolerant when run manually in one's own project though. Ideally, Gazelle should accept a flag (which is only provided in go_repository) which would turn off exit codes for non-fatal errors.

[jayconrod]

Migrated to bazelbuild/bazel-gazelle#60