Fix another case of exception swallowing from PyObject_GenericGetAttr

[mdickinson]

When an attribute is looked up on a CTrait object, None is returned on failed lookup, regardless of the actual exception that was raised. This means that (for example) logic errors from a CTrait method will be silenced.

This PR changes the behaviour to only return None if the attribute lookup raises AttributeError, and to propagate any other exception.

This is the cTrait analog to #959 for CHasTraits.

Along with #959, this fixes #946.

[mdickinson]

Here's an example of a (desirable, IMO) behaviour change due to this PR

On master:

>>> from traits.api import *
>>> ct = CTrait(0)
>>> ct.set_default_value(DefaultValue.list_copy, 1j)  # Bad: 2nd argument should be a list
>>> ct.default
>>> ct.default is None
True

On this branch:

>>> from traits.api import *
>>> ct = CTrait(0)
>>> ct.set_default_value(DefaultValue.list_copy, 1j)
>>> ct.default
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mdickinson/Enthought/ETS/traits/traits/ctrait.py", line 71, in default
    return value[:]
TypeError: 'complex' object is not subscriptable

OTOH, with DefaultValue.dict_copy in place of DefaultValue.list_copy, the exception gets swallowed again, because an AttributeError is raised. :-(

[corranwebster]

LGTM