This patch adds more friendly error messages for three common classes of
assertion:
    - missed sequence numbers due to being griefed by another thread
    - unknown requests in queue due to being griefed by another thread
    - extensions dequeuing too much or too little reply data

It adds error messages offering advice (e.g. call XInitThreads() first)
on stderr, but still generates actual assertions.  Hopefully this means
it's a little more Googleable and a little less frightening.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>