Major revision for error handling in the interpreter

[John W. Eaton]

With the changeset

  http://hg.savannah.gnu.org/hgweb/octave/rev/fd32c1a9b1bd

the "error" function (and similar functions that ultimately call it), 
simply throws an exception that contains information about the error 
(message, id, stack info) instead of printing an error message 
immediately and then throwing an exception.

It is now the responsibility of any code that catches an execution 
exception to determine whether and when to display error messages.  I 
believe that I have made all the necessary changes so that errors are 
still displayed where they should be, but I may have missed something. 
If you notice that error messages are not displayed correctly, please 
submit a bug report for the problem.

The new approach is more flexible and allows for some simplification of 
the error message routines as they no longer need feedback from the 
interpreter to know when to display or buffer messages.

jwe