Improve error message if two tasks from the same workflow attempt to enter the same group concurrently

Current situation

The message is quite terse ('same workflow as running or pending workflow)

Desired outcome

Make it less terse.

Running task in group "{name}" ({group.running}) has the same parent workflow as task requesting entry ({task}).  You cannot have two task belonging to the same workflow in the same group at the same time.  Cancelling.