If this were Java, we'd be writing:

   class ErrorException extends Exception ...;
   class NoSourceFileErrorException extends ErrorException ...;
   ...;

and then

   try {
     ...
   } catch (ErrorException e) {
     System.out.println (e.toString ());
   }

but it isn't so we can't.

The attached patch establishes a framework that makes it possible to 
work around this limitation of C.  It adds:

	struct exception;
	struct exception catch_exception(...)
	throw_exception(struct exception)

(The existing throw_exception is renamed to throw_reason to avoid a name 
clash, callers updated).   This lets users write a rough equivalent:

   struct exception e = catch_exceptions (....)
   switch (exception.reason)
     ...
     case ERROR_RETURN:
       print (exception.message);

and even opens the possibility of:

   TRY_CATCH (e)
     {
       ...
     }
   switch (e.reason)
     ...

The hacks error_silent and error_last_message can now be eliminated.

exceptions.[hc]'s has been rewritten to use it; tested on ppc gnu/linux.

committed,
Andrew