report a better position for // comment errors

addresses #147

plus it does the same to the errors where a comment cannot start with an uppercase letter.

had to inline the `startsWithLowercase()` to remember the position of the first \w character.
i don't like that we're moving this block of code from a function and making it inline. makes it much harder to read. why are we just not calling createFailure with an offset of 2?