Firebase Auth: Exception when signup has been blocked by cloud function is too generic

### [READ] Step 1: Are you in the right place?

I think so.

### [REQUIRED] Step 2: Describe your environment

* Firebase Component: Auth
* Component version: 21.3

### [REQUIRED] Step 3: Describe the problem

When the signup is blocked by the cloud function it returns a rather generic FirebaseException with the message:
```
An internal error has occurred. [ BLOCKING_FUNCTION_ERROR_RESPONSE:HTTP Cloud Function returned an error: {"error":{"message":"Signup with provider [google.com](http://google.com/) not allowed","status":"PERMISSION_DENIED"}} ]
```

This should be either a specific Exception (like FirebaseAuthBlockingFunctionErrorException) or a FirebaseAuthException with a proper code set.

Login logic shouldn't have to parse the returned exception message for magic strings or have to parse JSON to be able to react to errors.

#### Steps to reproduce:

1. Set up Firebase Auth with a [blocking cloud function](https://firebase.google.com/docs/functions/auth-blocking-events?gen=2nd)
2. Sign up with a data that matches the blocking criteria

#### Relevant Code:
-