Make the qa tool more robust

[simoncozens]

This improves the robustness of ghtools.qa in a couple of ways:

• Ensures that the fontbakery report is posted as a GH comment just after fontbakery runs, so it is encapsulated from other methods.
• Wraps each function in a decorator which catches exceptions so that an exception in one test does not crash the whole tool.
• Posts a notification of any exceptions/failures to GH.

This is split into fairly small self-contained commits, so may be easiest to review one commit at a time.

[felipesanches]

ok. That's all looking great!

[simoncozens]

Cool, thank you!