Improve qb validation

fixes some bugs in the tests, also fixes https://github.com/doctrine/phpcr-odm/issues/541

note ``Doctrine\Tests\ODM\PHPCR\Query\Builder\BuilderConverterPhpcrTest::testOrderBy`` is failing .. but honestly I don't really think such a test makes sense. It is way too complex to test as it requires so much mocking. IMHO we are better off testing this via the various existing functional test. So I vote for removing that test.

/cc @dantleech 

should we have a ``__toString()`` in all the operands to make it easier to construct error messages? obviously we are leaking internals here that the user doesn't know about .. but leaving it out is likely even less helpful.