try to improve query.join() when dealing with pure SQL expressions

at least have it barf more elegantly, perhaps:

class A(Base):
    __tablename__ = 'a'
    id = Column(Integer, primary_key=True)
    data = Column(String)
    def __repr__(s):
        return '<A id:%s data:%s>' % (s.id, s.data)

class B(Base):
    __tablename__ = 'b'
    id = Column(Integer, primary_key=True)
    data = Column(String)
    def __repr__(s):
        return '<B id:%s data:%s>' % (s.id, s.data)

with some data:

session.add_all([A(data='a1'),A(data='a2'),B(data='b1'),B(data='b2'),](A(data='a1'),A(data='a2'),B(data='b1'),B(data='b2'),))
session.commit()

and subqueries:

subqa = session.query(A.id, A.data.label('blah')).subquery()
subqb = session.query(B.id, B.data.label('blah')).subquery()

First attempt:

session.query(subqa.c.blah, subqb.c.blah)

as expected this give a cross join of all A's and B's

Second attempt:

session.query(subqa.c.blah, subqb.c.blah).join((subqb,subqb.c.id==subqa.c.id
))

gives and error:
AttributeError: 'NoneType' object has no attribute 'base_mapper'

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