firebase-ios-sdk

extract errorMessage directly from response when server returns 200 (#2542)

* extract errorMessage directly from response in case of returnIDPCredential=YES

Fix #2522.

In this case, the server returns an 200 without an error like the following:

```
[response data:] {
  "error": {
    "code": 400,
    "message": "FEDERATED_USER_ID_ALREADY_LINKED",
    "errors": [
      {
        "message": "FEDERATED_USER_ID_ALREADY_LINKED",
        "domain": "global",
        "reason": "invalid"
      }
    ]
  }
}
```

Rather, it is directly enclosed in the response with an errorMessage without a structure:

```
 [response data:] {
  key: value
 ...
  "errorMessage": "FEDERATED_USER_ID_ALREADY_LINKED",
  "kind": "identitytoolkit#VerifyAssertionResponse"
}
```

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