Add some kind of error handler to Calc so we can trap / handle errors if desired, instead of just halting always

[rlwhitcomb]

Couple of problems:

• Syntax errors will terminate the parser, so that won't work to trap those
• Several different kinds of exceptions are thrown during execution, so are there different kinds of handling?
• Don't even know how to handle continuing if we simply trap exceptions -- we'll have to decide before the "throw" what we're going to do, which is VERY complicated for exceptions thrown from the lower levels