Raise exception with helpful error on Client pickle.

[lukesneeringer]

Based on the discussion in #3191, raise an exception with a helpful error stating that Client classes are not pickleable.

Note to self: We probably want this in GAX also.

[dhermes]

_PICKLE_MSG = """\
Pickling client objects is explicitly not supported.

Clients have a large amount of state that is local and unpickleable.
"""

class Client(...):
    ...
    def __getstate__(self):
        raise NotImplementedError(_PICKLE_MSG)