use enum ParseError for errors instead of strings

[seanmonstar]

Strings make things harder. When trying to decide if my error_handler should eat the error or not, the compiler won't notice if I typo an error message. And if the message changes, the compiler won't warn me about a non-existent enum variant.

With an enum, I can match for certains errors. I made the Show implementation mirror the text of the errors from before, in case they were used to show in a Browser Console or something. This also allows for the messages to be adjusted slightly, while leaving the variant alone.

[SimonSapin]

Thanks for contributing! Please add #[deriving(PartialEq, Eq, Clone)] on pub enum ParseError. r+ with that, assuming it fixes the Travis build.

[seanmonstar]

Ah sure, I didn't realize that was additional tests beyond cargo test. I've added those deriving, and also noticed that ErrorHandler and parse_error were still using &str, so I converted all those into variants as well.

./doc.sh --test passed locally, so I believe travis should pass.

[SimonSapin]

Merged, thanks!