wintip-993985: Fix broken tooltip in Firefox for windows

We were using OS.Path.dirname, but that (reasonably) assumes '\' on windows,
and when you're manipulating URL segments, you don't want that, so this uses a
dirname based on '/' everywhere.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
wintip-993985: Add on-error handler to load promises

Signed-off-by: Joe Walker <jwalker@mozilla.com>
wintip-993985: Add on-error handler to load promises

Signed-off-by: Joe Walker <jwalker@mozilla.com>
Merge branch 'master' into mozmaster