Add handling of Dwolla::RequestException

By wrapping and re-raising the `Dwolla::RequestException`
as a `CallbackException`, OmniAuth OAuth2 handles it well
(as an 'invalid credentials' failure) and allows the app's
authentication/authorization engine to process as needed.

For example, in the case of Devise, the default behavior is to
fail user authentication.  This can easily be adjusted based
on the wrapped exception to, for example, let the user know
that Dwolla payment permission was not received.

Without this change, the original `Dwolla::RequestException`
is passed up the chain.  For OAuth-based applications, the
exception is never handled in the OAuth layer.  In the case of Devise, this means the exception causes the application to
have an un-handled error, rather than refusing authentication as one would expect.