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"
}
```

Author: Yue-Wang-Google

Firebase/Auth/Source/RPCs/FIRAuthBackend.m

@@ -128,6 +128,12 @@
  */
 static NSString *const kErrorMessageKey = @"message";
 
+/** @var kReturnIDPCredentialErrorMessageKey
+    @brief The key for "errorMessage" value in JSON responses from the server, In case
+        returnIDPCredential of a verifyAssertion request is set to @YES.
+ */
+static NSString *const kReturnIDPCredentialErrorMessageKey = @"errorMessage";
+
 /** @var kUserNotFoundErrorMessage
     @brief This is the error message returned when the user is not found, which means the user
         account has been deleted given the token was once valid.
@@ -925,18 +931,15 @@ - (void)postWithRequest:(id<FIRAuthRPCRequest>)request
     if ([request isKindOfClass:[FIRVerifyAssertionRequest class]]) {
       FIRVerifyAssertionRequest *verifyAssertionRequest = (FIRVerifyAssertionRequest *)request;
       if (verifyAssertionRequest.returnIDPCredential) {
-        NSDictionary *errorDictionary = dictionary[kErrorKey];
-        if ([errorDictionary isKindOfClass:[NSDictionary class]]) {
-          id<NSObject> errorMessage = errorDictionary[kErrorMessageKey];
-          if ([errorMessage isKindOfClass:[NSString class]]) {
-            NSString *errorString = (NSString *)errorMessage;
-            NSError *clientError = [[self class] clientErrorWithServerErrorMessage:errorString
-                                                                   errorDictionary:errorDictionary
-                                                                          response:response];
-            if (clientError) {
-              callback(clientError);
-              return;
-            }
+        NSString *errorMessage = dictionary[kReturnIDPCredentialErrorMessageKey];
+        if ([errorMessage isKindOfClass:[NSString class]]) {
+          NSString *errorString = (NSString *)errorMessage;
+          NSError *clientError = [[self class] clientErrorWithServerErrorMessage:errorString
+                                                                 errorDictionary:@{}
+                                                                        response:response];
+          if (clientError) {
+            callback(clientError);
+            return;
           }
         }
       }