Revert "x11: don't set global error handler"

This reverts commit 877303aaa9111fc56a8e5edbeb439699acfe44c0.

The OpenGL 2.1 fallback for vo_opengl didn't work. Two things come
together: 1. trying to create an OpenGL 3.0 context will fail with
a GLXBadFBConfig error, and 2. X errors are fatal by default. Since
the reverted commit removed the X error handler, the mpv process was
killed, instead of continuing for the fallback.

(Note that this commit is not an exact inverse commit, since mp_msg
changed, but it does about the same thing.)