Add sanity checks for permissions

[mabrikan]

Fixes #278.

browser.notifications.create is undefined when notification permission is not granted.

This makes calls to function showNotifications fail stopping the program flow and preventing saving lastModified field.

Also perhaps we need to unset permission-related options whenever a permission is removed.
onRemove[1] listener is a good approach.

[1]
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/permissions/onRemoved

[notlmn]

Also perhaps we need to unset permission-related options whenever a permission is removed. onRemove[1] listener is a good approach.

[1] developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/permissions/onRemoved

I'm not sure if we can keep the options storage and permissions from the browser side in sync. The best approach as you have documented here is to always check for permissions before performing any action.

[notlmn]

Thanks for the CL 🙌