Bug 1412856 part 6. Add more useful error messages for serviceworker exceptions. r=dom-workers-and-storage-reviewers,perry?

For some of these I had to take a guess due to lack of familiarity with this
code, so careful review of the actual strings would be much appreciated.

There's still a bunch of code in dom/serviceworkers that constructs a
CopyableErrorResult from just an nsresult, but I don't understand that code well
enough to write good error messages.

Differential Revision: https://phabricator.services.mozilla.com/D61201