Return connection failure if display string specifies non-existent screen

Return connection failure if display string specifies non-existent screen

Matches the behaviour of Xlib - if you set DISPLAY to :0.1 but only have
one screen, closes connection and returns error.

This introduces a new connection error code:
XCB_CONN_CLOSED_INVALID_SCREEN

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>