Print out counter error message. (#5552)

* Print out counter error message.

* style

Author: mikehaney24

GoogleDataTransport/GDTCORLibrary/GDTCOREvent.m

@@ -56,7 +56,8 @@ + (NSNumber *)nextEventID {
                            atomically:YES
                              encoding:NSUTF8StringEncoding
                                 error:&error];
-    GDTCORAssert(error == nil, @"There was an error saving the new counter value to disk.");
+    GDTCORAssert(error == nil, @"There was an error saving the new counter value to disk: %@",
+                 error);
   });
   return result;
 }