Pattern: Return errors instead of null

 As Taverna can't (currently) handle null (see http://www.mygrid.org.uk/dev/issues/browse/TAV-653) - in Taverna 2 one can instead return exceptions on individual ports and inside lists.

Exceptions are registered as error documents by Taverna, and are passed along. The ErrorBounce layer of processors downstream will prevent execution if they see this 'null'-exception - as showed in this example that the string concatination is just run for the two list elements that are not exceptions. Downstream processors that don't read the port with an error on it's output are not affected.

This pattern could be used for instance for database lookups. A database NULL has semantically almost the same meaning as error documents in Taverna - placeholders for the expected value. 

TAV-653 suggests registering a special exception that is defined as null, so that future null-supporting services/beanshell scripts can dereference the error as null again - while services that can't handle it will behave like in this workflow.