Fix wrong status code if context.abort has been called

[thangtp]

When context.abort is invoked prior to the interceptor being reached, the status code specified in context.abort should be used, taking precedence over status_on_unknown_exception.

Example:

Given the use of the following interceptor:

ExceptionToStatusInterceptor(status_on_unknown_exception=grpc.StatusCode.INTERNAL)

If context.abort is implemented somewhere in the service as:

context.abort(grpc.StatusCode.RESOURCE_EXHAUSTED, 'resource exhausted')

Expected Behavior:

The service should return an error with the status code set to grpc.StatusCode.RESOURCE_EXHAUSTED.

[d5h]

Awesome! Thank you for this contribution @thangtp! Much appreciated. The release should go out shortly.