Make AssociationReflection#reciprocal not raise error if associated class contains association with invalid associated class

Previously, if model A has an association with model B, and
any associations in model B reference an invalid class,
calling reciprocal on the association reflection between model
A and model B would raise a NameError.

While this is good in that it alerts you to an error, I don't
think it's reciprocal's place to do so.

I'm on the fence about whether this is a good idea in general,
but for backwards compatibility this should probably go in. The
changes made since 4.14.0 make this more likely to happen in user
code.
Bump version to 4.15.0