Change assert to exception in get_queue

It would be useful for PyViennaCL for this assert to be an exception instead. I've hijacked the invalid_command_queue exception (which seems appropriate, if not technically correct), but anything would probably do. The problem with assert is that it causes Python to die.

I had a look at systematically changing all the asserts in ViennaCL to exceptions, but as there are more than 500 of them, that's probably a project for another day..