Nested exceptions from failed PrefixList validation.

[mdickinson]

Failed assignments to PrefixList cause a nested exception: we raise a TraitError, catch that raised error, and then raise another TraitError. This suggests a logic error somewhere.

>>> from traits.api import *
>>> class A(HasTraits):
...     foo = PrefixList(["one", "two", "three"])
... 
>>> a = A()
>>> a.foo = "t"
Traceback (most recent call last):
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 2526, in validate
    self.error(object, name, value)
  File "/Users/mdickinson/Enthought/ETS/traits/traits/base_trait_handler.py", line 76, in error
    raise TraitError(
traits.trait_errors.TraitError: The 'foo' trait of an A instance must be 'one' or 'two' or 'three' (or any unique prefix), but a value of 't' <class 'str'> was specified.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 2530, in validate
    self.error(object, name, value)
  File "/Users/mdickinson/Enthought/ETS/traits/traits/base_trait_handler.py", line 76, in error
    raise TraitError(
traits.trait_errors.TraitError: The 'foo' trait of an A instance must be 'one' or 'two' or 'three' (or any unique prefix), but a value of 't' <class 'str'> was specified.