Correct exception thrown attempting to access a non-existent C/C++ global variable on the 'cvar' object.

The exception thrown used to be a NameError. However, as this access is
via a primary, an AttributeError is more correct and so the exception
thrown now is an AttributeError. Reference:
http://docs.python.org/2/reference/expressions.html#attribute-references

SF Patch #346.

Examples/test-suite/global_vars.i

@@ -28,4 +28,9 @@
   Hello h;
   Hello *hp;
   Hello &hr = h;
+
+  void init() {
+    b = "string b";
+    x = 1234;
+  }
 %}