fixes #208

We can't rely solely on the number of the returned messages
because it could be zero if all the event types are unknown.

fixes #168

fixes #87

fixes #184
fixes #209

This can happen when a new room is being created from a different client and thus is created
with no known state to the local client.

Show a better message on the login screen after an initial sync failure.

fixes #181

fixes #216

fixes #86
fixes #89