Avoid allocating Lisp data from code that can be run from a signal handler.

Avoid allocating Lisp data from code that can be run from a signal handler.
(x_error_message): New var to replace x_error_message_string.
(x_error_catcher, x_catch_errors, x_catch_errors_unwind)
(x_check_errors, x_had_errors_p, x_clear_errors, x_error_handler)
(syms_of_xterm): Use it instead of x_error_message_string.