python: provide better error message for wrapped C++ methods

If you successfully export a C++ SimObject method, but try to
invoke it from Python before the C++ object is created, you
get a confusing error that says the attribute does not exist,
making you question whether you successfully exported the
method at all.  In reality, your only problem is that you're
calling the method too soon.  This patch enhances the error
message to give you a better clue.

src/python/m5/SimObject.py

@@ -724,9 +724,15 @@ class SimObject(object):
         if self._ccObject and hasattr(self._ccObject, attr):
             return getattr(self._ccObject, attr)
 
-        raise AttributeError, "object '%s' has no attribute '%s'" \
+        err_string = "object '%s' has no attribute '%s'" \
               % (self.__class__.__name__, attr)
 
+        if not self._ccObject:
+            err_string += "\n  (C++ object is not yet constructed," \
+                          " so wrapped C++ methods are unavailable.)"
+
+        raise AttributeError, err_string
+
     # Set attribute (called on foo.attr = value when foo is an
     # instance of class cls).
     def __setattr__(self, attr, value):