Fix two crashes when a display connection is lost

Fix two crashes when a display connection is lost

This fixes errors caused by invalid error traps being left on
the error handler stack if an IO error causes a non-local exit
out of the protected code, and another crash caused by
delete_frame trying to read async input.

* src/eval.c (unwind_to_catch, push_handler_nosignal): Save and
restore the X error handler stack.
* src/lisp.h (struct handler): [HAVE_X_WINDOWS]: New field
`x_error_handler_depth'.

* src/xterm.c (struct x_error_message_stack): Make string a
regular string.
(x_unwind_errors_to): New function.
(x_error_catcher, x_catch_errors_with_handler)
(x_uncatch_errors_after_check, x_uncatch_errors): Update the
stack depth.
(x_check_errors): Stop manually unwinding since unwind_to_catch
now does that for us.
(x_had_errors_p, x_clear_errors): Update for new type of
`string'.
(x_connection_closed): Block input between just before
delete_frame to when the terminal is unlinked.

* src/xterm.h: Update prototypes.