Fix call to jabber-activity-mode-line-update

It doesn't take any arguments anymore.
Improve error message for connection failure

Ensure that the final message, the one that stays in the echo area,
contains information about where we tried to connect to.  Also
accumulate error messages for all addresses we tried.
Don't report nil resource when connection lost

If we didn't get a resource before the connection was lost, don't put
"/nil" in the error message, e.g. "foo@example.com/nil: connection lost".