Add two compound URL invalidity cases for isValid()

Add two compound URL invalidity cases for isValid()

These two errors can only happen if one calls setPath() explicitly. They
cannot happen for parsed URLs, which is why they are only caught with
isValid(). It's not possible to set the error condition in setPath()
either because they depend on the presence / absence of the authority
and scheme.

Also update all the unit tests that set a path not starting with a slash
and were just "freeloaders" on the previous behaviour.

Change-Id: Ice58cd4589a850452d7573a5b19667bbab2fb43e
Reviewed-by: David Faure <faure@kde.org>