Better error checking on private key and csr.  Also, pull in file names ...

...instead of pointers.
Also, you should be raising `argparse.ArgumentTypeError` instead of `IOError` as per https://docs.python.org/2.7/library/argparse.html#type - this will be nicely displayed to the CLI and you can remove the `try: args = parser.parse_args() except IOError` block