Improved error handling

[stenlarsson]

Consider the following expression:

`foo`

This expression contains an invalid json-value, and this will raise the unhelpful error unexpected token 0. The reason for the zero is that Symbol#match surprisingly returns a fixnum, unlike String#match. Still, unexpected token unknown would not be much better, therefore unknown tokens raise a separate error, in this case unknown token "foo".

(Note that this particular expression is accepted by other implementations, but I don't think it should be according to the specification.)

[trevorrowe]

Sorry for the slow response. Thank you for the fix. This should be part of the 1.3 release shortly.