stats: Add more information to uninitialized error

ClockedObject was changed to require its regStats() to be called from every
child class. If you forget to do this, the error was indecipherable. This
patch makes the error more clear.

src/base/statistics.cc

@@ -221,7 +221,9 @@ Info::baseCheck() const
 #ifdef DEBUG
         cprintf("this is stat number %d\n", id);
 #endif
-        panic("Not all stats have been initialized");
+        panic("Not all stats have been initialized.\n"
+              "You may need to add <ParentClass>::regStats() to a"
+              " new SimObject's regStats() function.");
         return false;
     }