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.
Why do we need `_` in attributes name ? Sounds weird to me as it is not standard... `-` would be recommanded wherever you are tempted to use an `_`