Correct exceptions for nested types

[krey]

from traitlets import HasTraits, Union, Unicode, Bool

class Foo(HasTraits):
    bar = Union([Bool(), List(Unicode())])

Foo(bar=[1, 2, 3])

TraitError: The 'bar' trait of a Foo instance must be a boolean or a list, but a value of [1, 2, 3] <type 'list'> was specified.

If you're not looking at the code for Foo, the error message isn't gonna tell you what's wrong

[rmorshea]

@krey this was fixed by #402 however it hasn't been released yet.