FR: Specify why auth/missing-identifier is being returned

### [REQUIRED] Describe your environment

* Operating System version: macOS 12.3
* Browser version: Firefox 111.0.1
* Node version: 16.17.0
* Firebase SDK version: 9.19.1
* Firebase Product: auth

### [REQUIRED] Describe the problem

I recently ran into an issue where auth/missing-identifier was being returned with no details anywhere on why this error was being returned. I learned later it was because I was passing a token that wasn't created through the Firebase Admin SDK but it wasn't clear when all I had to go on was `Firebase: Error (auth/missing-identifier).` and there wasn't any explanation anywhere in https://firebase.google.com/docs.

#### Steps to reproduce:
To reproduce the issue, just call `signInWithCustomToken` with a valid JWT token that wasn't generated through Firebase Admin.
#### Relevant Code:

```javascript
// jwtToken is a JWT token that wasn't generated through Firebase Admin
await signInWithCustomToken(auth, jwtToken);
```