Create arguments_valid flag

The argument handling code previously would exit any time an invalid
argument was seen in some level of the code.  This seemed like the
right behavior at the time, but it slowly grew to making the code
untestable.

This commit creates an arguments_valid flag, which is set when a bad
argument is encountered, instead of just exiting immediately.

This makes the code easier to test, which is necessary for future
modifications to the argument handling code.
Update module name for in unit tests.

The module name for the code under test in
test_single_pull_arg_handler.py was misspelled.  This is a sign of a
growing problem: I don't have an automated way to run ALL test suites.

This commit changes the unit test module to run (and pass), but the
underlying problem is still an issue and needs addressed.