pyctest does not handle `None` gracefully

[carterbox]

I forgot to install nose, which means that pyctest was passed a command list which included None.

    # test properties
    props = {
        "DEPENDS": "correct_module",
        "RUN_SERIAL": "ON",
        "LABEL": "unit",
        "WORKING_DIRECTORY": binary_dir,
        "ENVIRONMENT": "TOMOPY_USE_C_ALGORITHMS=1"
        }

    # test command: python $(which coverage) run $(which nosetest)
    cmd = ['/opt/miniconda/envs/travis/bin/python', None, 'run', None]

The resulting RuntimeError was esoteric.

Error running pyctest - Unable to cast Python instance to C++ type (compile in debug mode for details)
Traceback (most recent call last):
  File "pyctest_tomopy.py", line 326, in <module>
    run_pyctest()
  File "pyctest_tomopy.py", line 307, in run_pyctest
    create_nosetest_test(args)
  File "/home/chingd/Documents/tomopy/benchmarking/__init__.py", line 244, in create_nosetest_test
    pyctest.test("nosetests", cmd, props)
RuntimeError: Unable to cast Python instance to C++ type (compile in debug mode for details)

It should probably return a TypeError with a message about how commands cannot be None.

[jrmadsen]

I appreciate you reporting this. That failed cast is (obviously, of course) happening in the C++ code but the lack of info is due to PyBind11 (not my code). However, I found some PyBind11 documentation about customizing exception handling but I currently can't really prioritize patching this due to more pressing things on my plate since the error is being detected and just not giving a whole lot of info, as opposed to not being detected (which would be much more concerning).

When I do get around to it though and if I find an elegant solution, I might actually just submit the change to PyBind11 itself.