run_script() handles 'No Processor Available'

Tricky to test, since you need for the scripting service to throw 'No Processor Available'. You can do this by adding ``` raise Exception("No Processor Available") ``` to the code in a couple of places (as I did).

Instead of giving a feedback dialog with exception, we now show a nice message in the Activities panel.

![screen shot 2013-08-30 at 13 43 37](https://f.cloud.github.com/assets/900055/1057865/0dceb42e-117d-11e3-97a4-a07022a03793.png)

See https://trac.openmicroscopy.org.uk/ome/ticket/10231 and https://github.com/openmicroscopy/openmicroscopy/pull/639/