Parser error-handling

[skx]

Every time a parse-function returns nil there should be a corresponding addition to p.Errors().

Otherwise the user will not receive the correct error-message. (Of course as a result of #144 only the first error will be shown by default, all other errors are accessible - but in practice they're a little bogus once the parse gets thrown off. The parser needs to resync, and restart parsing after skipping to the end of the statement. Without a terminating ; that becomes a tricky thing to do though.)

[skx]

153 improved this a little. There is probably more room to improve, as the parser has only 87% test-coverage. I'll work on that shortly.