Add support for clearing the error status

Add a slot to the structure, and a macro to the keypress form
execution context, to allow the clearing of the error state on the
stack.
Allow clx to clear errors

The "clx" operation should clear the error state.  This change does
that.
Code for error display and input editing

Now allow the screen to display error messages when something is done
incorrectly.  Also allow the Rubout key to work at the command line.
Numeric input hardening and error status code

Make sure that the numeric input code doesn't encounter bad input that
causes it to exit to the Lisp debugger.

Also ensure that no operations have any effect when the stack is in an
error status, except those operations that are able to clear the error.