* src/xterm.c (struct x_error_message_stack): Add new fields for a
callback function and associated data pointer.
(x_error_catcher): If the callback function is set, call it after
saving the error message string.
(x_catch_errors_with_handler): Renamed from x_catch_errors but now
accepts a callback function and data pointer.
(x_catch_errors): Now a wrapper function.
* src/xterm.h (x_special_error_handler): New typedef.
(x_catch_errors_with_handler): Declare.