NameError: name 'FileNotFoundError' is not defined when a specific browser is chosen

In Python <= 3.2 Popen throws `OSError`. Starting with Python 3.3 it throws `FileNotFoundError` which is a subclass.

isrcsubmit expects FileNotFoundError, leading to an uncatched `NameError` instead on Python <= 3.2.

The impact is quite small, because the problem only shows up when a browser is explicitely set as parameter or in the config *and* it is not found.
And in that case it fails either way, but failing with an error message is better than with this NameError.

The problem was originally found by Jozo with PyFlakes.