Validate input args in luigi.run()

[subhramit]

Description

Validate passed args in run() before passing anything downstream.

Motivation and Context

Closes #3354

Quoting from the linked issue:

luigi.run() should validate that the input args is an iterable and raise a meaningful TypeError or ValueError such as:TypeError: luigi.run() expects a list or tuple of command-line arguments, got int.

This adds the above validation.

Have you tested this? If so, how?

I added unit tests and ran them.

[subhramit]

Hi, I'm not sure what other tests to add to satisfy codecov.

Edit - after a while, it passed on its own somehow.