bookmarks-import: Fix error handling

We need to, at minimum, use g_clear_error() to ensure it is NULL before passing it to g_set_error(). But it's simpler to use a separate GError for internal errors instead.

Now instead of crashing, we'll show a nice error message "Firefox bookmarks could not be retrieved!" This isn't ideal, however, because clearly the code expects to trigger the earlier error message ("Close Firefox and try again") but is failing to do so.

Fixes #965 (closed)