Implement schema abstraction api and error handling

This MR adds several schema abstraction classes to simplify interactions.

Requests are now rejected when receiving api errors, returning the error code, type and message.

This also fixes the import statement for the library. Users can now use @cubicweb/client instead of @cubicweb/client/lib.

Authentication does not yet work out of the box. Users need to log in to the cubicweb interface and change the auth_tkt Same Site restriction from lax to no restriction.