Fix error message on provisioning timeout

"Coroutine raised StopIteration" is pretty hard to interpret as timeout and stuck provisioning VM.

Jan 15 20:39:22 coin-master coin[93612]: ERROR:workitem.py:445(93673): Houston? Houston! Unhandled exception occurred while running a WorkItem. This is potentially serious! Investigate it immediately.
Jan 15 20:39:22 coin-master coin[93612]: Traceback (most recent call last):
Jan 15 20:39:22 coin-master coin[93612]:   File "/var/lib/coin/tqtc-coin-ci/src/workitem.py", line 945, in run
Jan 15 20:39:22 coin-master coin[93612]:     result = next(iter(done)).result()
Jan 15 20:39:22 coin-master coin[93612]:              ^^^^^^^^^^^^^^^^
Jan 15 20:39:22 coin-master coin[93612]: StopIteration
Jan 15 20:39:22 coin-master coin[93612]: The above exception was the direct cause of the following exception:
Jan 15 20:39:22 coin-master coin[93612]: Traceback (most recent call last):
Jan 15 20:39:22 coin-master coin[93612]:   File "/var/lib/coin/tqtc-coin-ci/src/workitem.py", line 393, in _run
Jan 15 20:39:22 coin-master coin[93612]:     await self.run()
Jan 15 20:39:22 coin-master coin[93612]: RuntimeError: coroutine raised StopIteration