raise error message when joining along two conflciting paths

[Mike Bayer]

e.g.:

Foo() -> 'bars' -> Bar() -> 'bats' -> Bat()
Foo() -> 'bats' -> Bat()

query(Foo).join(['bats']('bars',)).filter(...).join('bats') # -> error

because no aliases are being used but the "Bat" table is being requested twice via different paths. this is a generalization of the fix in #687.

[Mike Bayer]

Query in 0.5 is much more open ended than this and its not appropriate to police user errors in SQL construction - their results will give them the answers on that. Only things that have the end effect of being "ignored", like #687, should have something indicated.

[Mike Bayer]

Removing milestone: 0.4.xx (automated comment)