Make doReturnResults more robust for CmdLineTask

If a command-line task fails and doReturnResults is True and doRaise is False
then an unhelpful exception occurs instead of returning the results.
This commit fixes the problem by setting the task result to None.
It also adds a unit test for the condition.
The unit test requires an updated obs_test (for its updated TestConfig).
Remove __all__ from testCmdLineTask

__all__ was exporting one symbol (TestConfig) that was not even defined
in the test. This is crazy and I can find no code that uses it.
If you need the TestConfig get it from obs_test.
Merge branch 'tickets/DM-1558'