Automatically fallback to jQuery mode if the SW fails to register.

[mossroy]

Fixes #405

[Jaifroid]

EDIT: This report shows the previous behaviour. It is now fixed by this PR.

Just tested on Firefox Quantum, Chromium and Edge:

• On Chromium and Edge, I get a broken and unusable app after switching to Service Worker mode when using the file:// protocol. I am allowed to switch to this mode, even thought the first API status is orange, and the mode remains selected, even thought the API remains orange. Switching back to jQuery mode does not correct the situation -- the app remains unusable until I reload.

• On Firefox Quantum, it also allows me to turn on Service Worker mode in file:// protocol, even though the first API status is orange, and it remains selected despite not working. However, switching back to jQuery mode allows the app to continue functioning as normal.

[Jaifroid]

I've now tested on all four main browsers (including IE), and this PR fixes the issue. It successfully prevents selection of SW mode if the browser doesn't support it in the file:// context (which is the context I tested).