JobState.INVALID is not handled properly

When a job is rejected by the server the JobStatus.INVALID is set when the job is polled but this is not handled by the UI. This currently affects all Slivka services. Jabaws is unaffected because instead a an exception is raised on submission which is interpreted as server error and reported to the user:

ClustalOWSClient: Failed to submit the sequences for alignment (probably a server side problem)
When contacting Server:http://www.compbio.dundee.ac.uk/jabaws

java.lang.Exception: Server at http://www.compbio.dundee.ac.uk/jabaws returned null object, it probably cannot be contacted. Try again later.
at jalview.ws.gui.MsaWSThread.StartJob(MsaWSThread.java:313)
at jalview.ws.AWSThread.run(AWSThread.java:117)