Cleanup binder_cache.py

[yuvipanda]

• Use argparse for argument parsing rather than reading from sys.argv directly.
• Print multi-line error messages to sys.stderr rather than raise exceptions. Since the goal is for the user to read them, rather than a system, this is cleaner.
• Use any() to check for existence of non-dockerfile files, rather than a list comprehension. This expresses intent better, and is more performant.
• Put everything into a function
• Add correct #! to tell linux what interpreter to use to execute this file

[hamelsmu]

👍🏽