Fix Exact Gateway test, handle 401 auth failure

Fixes test failure:
RemoteExactTest#test_failed_capture, by testing for correct response string

And test error:
RemoteExactTest#test_invalid_login, by properly handling 401

I'm really concerned that this loses whatever's in the body of the error (we're not guaranteed an invalid login is the only thing that can cause a 401); can you put the response body into the exception message maybe? Also it should be "login" not "logon".