Fix blank error code

Author: Junio Branco

ios/Firestack/FirestackAuth.m

@@ -196,11 +196,11 @@ @implementation FirestackAuth
          if (user != nil) {
              [self userCallback:callback user:user];
          } else {
-             NSDictionary *err = @{
-                                   @"error": @"createUserWithEmailError",
-                                   @"name": @([error code]),
-                                   @"description": [error localizedDescription]
-                                   };
+             NSDictionary *err =@{
+                                  @"error": @"createUserWithEmailError",
+                                  @"code":  [NSString stringWithFormat:@"%d", @([error code])],
+                                  @"message": [error localizedDescription]
+                                  };
              callback(@[err]);
          }
      }];