fix #225 Improve compiler error for attributes with missing quotes

This PR adds error messages for wrong attributes of HTML elements and components.
Please have a look at the tests first.

It doesn't takes care of allowing single quotes, @olaf-k is working on this part.
Just a note, I haven't read the whole commit, but first thing I noticed is that `_elementOrComponent` becomes a 160-liner after this change. Maybe it would make sense to split some parts of it using helper functions? (either now or in a separate refactor commit).