catch if #watch_src goes funny.
TODO
Catch exceptions on @thread.run - much less annoying now, hacky as things are.
experimental changes to see if we can tame the threads.
Revert "experimental changes to see if we can tame the threads."

This reverts commit 8af11314313711b0b7654f3440183c9bd2a7e15a.
found the right choke point to evaluate and report exceptions.