If cigetcert fails for any reason, raise PermissionError

[shreyb]

Fixes #570.

This PR also adds two new unit tests to check the new logic, as well as a minor bug fix for the existing getProxy tests themselves.

Note: According to the docs:

• https://docs.python.org/3.7/library/subprocess.html#subprocess.run
• https://docs.python.org/3.7/library/subprocess.html#subprocess.CompletedProcess.check_returncode

there are two ways to check if a subprocess.run() call fails:

1. Pass the check=True kwarg to the function, which will make it raise a CalledProcessError if the return code of the underlying subprocess is nonzero (i.e. subprocess.run('exit 1', check=True) will raise a CalledProcessError), and
2. If you don't want to immediately raise the error, set check=False, and then save the return value of subprocess.run() to a variable (e.g. result). Then, when you're ready, you can run result.check_returncode(), which will raise the CalledProcessError if the return code is nonzero.

We use the second method here, since we want to raise different errors based on the output of cigetcert.

[goodenou]

These changes look good Shreyas. Good ahead and merge the PR.

[shreyb]

Merging, as Lisa gave her written approval in the comment above.