In the case of OAuth errors which we get through the message
channel, throwing an exception doesn't help us as the init function
will not be aware about it.
So for now, I've moved the nodejs style error handling in this
particular case.
For now I've mostly tried to follow the same style guide as the flutter
repository, with many options disabled. Eventually, maybe it would make
sense to be far stricter.
It's so awesome that I don't need to store the client secret with
GitLab. The GitHub APIs are so much easier to use + documentation, but
at least GitLab supports more OAuth methods.
