stats: better error message for uninitialized statistic

As suggested by Nathan Binkert in 2008:
http://permalink.gmane.org/gmane.comp.emulators.m5.users/2676

src/python/m5/stats/__init__.py

@@ -67,7 +67,8 @@ def enable():
 
     for stat in stats_list:
         if not stat.check() or not stat.baseCheck():
-            fatal("stat check failed for '%s' %d\n", stat.name, stat.id)
+            fatal("statistic '%s' (%d) was not properly initialized " \
+                  "by a regStats() function\n", stat.name, stat.id)
 
         if not (stat.flags & flags.display):
             stat.name = "__Stat%06d" % stat.id