pep8analysis: rename its Error class to P8Error.

One day, name conflicts will be resolved sanely... one day...

Signed-off-by: Jean Privat <jean@pryen.org>
lib: add standard/error.nit

This could help people to implement some error handling in libraries.

It is inspired by #701 and ideas from Alexandre Terrasa.

Signed-off-by: Jean Privat <jean@pryen.org>