Warning box when starting an invalid task.

Previously this would silently fail to start the task.

Fixes https://github.com/KDAB/Charm/issues/171

Change-Id: I1386b8eb4d2a680cadd72d2abeea258721391609
Reviewed-on: https://codereview.kdab.com/10218
Reviewed-by: Continuous Integration <build@kdab.com>
Reviewed-by: Frank Osterfeld <frank.osterfeld@kdab.com>