'swig_varlink_getattr' throws a wrong exception

If the attribute of a python object could not found a AttributeException
should be thrown instead of a NameException.
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.