ASGARD-1219 - Fix NullPointerException in /show/task/123doesntexist

Added unit test for "not found" case.

The error message won't match logically for the workflowId/runId case, but we plan to remove that case soon anyway.
Merge pull request #369 from joesondow/ASGARD-1219-Fix-Task-NPE

ASGARD-1219 - Fix NullPointerException in /task/show/123doesntexist