Python3: stbt.detect_motion: Don't let `StopIteration` bubble up
Python3: stbt.detect_motion: Don't let `StopIteration` bubble up

In Python 3.7, an uncaught StopIteration inside a generator function
will be converted automatically to a RuntimeError. See [PEP 479].

[PEP 479]: https://www.python.org/dev/peps/pep-0479/

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester