fix(crashlytics, iOS): reorder error reason logging to match Android implementation

packages/firebase_crashlytics/firebase_crashlytics/ios/firebase_crashlytics/Sources/firebase_crashlytics/FLTFirebaseCrashlyticsPlugin.m

@@ -126,10 +126,10 @@ - (void)recordError:(id)arguments withMethodCallResult:(FLTFirebaseMethodCallRes
   }
 
   if (![reason isEqual:[NSNull null]]) {
-    reason = [NSString stringWithFormat:@"%@. Error thrown %@.", dartExceptionMessage, reason];
     // Log additional custom value to match Android.
     [[FIRCrashlytics crashlytics] setCustomValue:[NSString stringWithFormat:@"thrown %@", reason]
                                           forKey:@"flutter_error_reason"];
+    reason = [NSString stringWithFormat:@"%@. Error thrown %@.", dartExceptionMessage, reason];
   } else {
     reason = dartExceptionMessage;
   }