If the client started and local storage did not contain a saved
last-viewed group ID, then `groups.load()` would attempt to call
`store.focusGroup(null)`. As a result subsequent calls to
`groups.focused()` would return null and code in various places in the
application expects that there will always be a focused group.

Fix the problem by not setting the focus group after groups are fetched
unless the previously-focused group was set and exists in the new
groups list.

Also improve the behavior of `store.focusGroup` by making it leave the
focused group untouched if the specified group has not been loaded, but
log an error so we'll know (via Sentry) if it happens again.

Fixes #750