This fixes the real-time sync for the user groups membership data
structure to work like these work in the rest of Zulip; now, edits
made in one browser are immediately and correctly reflected in other
browsers.
Using user_group_name_dict.get() will return `undefined`.
`blueslip.error` statement caused an exception notification to the
admins.
Tweaked by tabbott to add a test for a nonexistent group.
