Add Exceptions

[gfwilliams]

Add JavaScript exceptions - we just need to find a nice way of doing this that doesn't slow us down when we're not using them.

[gfwilliams]

I've just pushed some code to handle Exceptions. It appears to work well enough now (at least for basic handling).

However: The interactive shell isn't aware of them, so doesn't display any errors. I think what's probably needed is:

• Merge EXEC_ERROR into EXEC_EXCEPTION, and make the JS libraries throw exceptions instead of errors
• Make sure the existing EXEC_ERROR handlers work ok in jsinteractive
• jsinteractive should also be aware of exceptions when executing code at the command-line (currently it relies on the code producing the error to output it to the console).
• Make code that previously output a stack trace to the console output it to the exception
• Cope with out of memory errors. Could be painful as how do we allocate an exception??

[gfwilliams]

Also see #316 - we should definitely throw reference errors