Raise exception with custom class

[bouk]

Hi there,

Currently, if no application default credentials were found, a RuntimeError with a constant error message is raised. This should be a custom class instead, so it can be caught in a rescue block.

This pattern is also done in web_user_authorizer.rb and user_authorizer

[BigTailWolf]

We are working in progress for throwing all exceptions as custom class from the library