Update comments in X error handling code

Update comments in X error handling code

* src/xterm.c (struct x_error_message_stack, x_error_message)
(x_error_catcher, x_catch_errors, x_had_errors_p)
(x_clear_errors): Long-overdue comment update.  Describe the
fields of the error message stack, and the fact that
`x_error_message' is a stack, not a pointer to a string (which
has been true since 2006).  Also describe the updated conditions
used to find an applicable error handler and to call
x_uncatch_errors_after_check.