- when the error received is of type `.internal`, the current code rewrites the message with `descriptionForErrorCode`. This is pretty bad because any contextual information we might have received from Firebase is destroyed.
- this fix attempts to extract the message from the response payload, preserving the original value.
- if message is not found, the message is set to the value returned by `descriptionForErrorCode`, as was done before.

…pb-internal-error-fix