BUG: Clear the exception when raising a TraitError.

When we call raise_trait_error(), we are usually doing so after some other
exception has been raised. In Python code, we would be catching this error
using `try: except: raise TraitError()`. In C, we use `PyErr_Clear();
raise_trait_error();`.
Merge pull request #119 from enthought/fix-clear-error

BUG: Clear the exception when raising a TraitError.