added clearer error message when doing many to many join without sails

[heyharvs]

When using waterline as a standalone, dominant flag will not fix the error unless the connection is also explicitly set.

[devinivy]

Can you give an example of what you mean by "unless the connection is also explicitly set."? When using waterline standalone, I think a connection always has to be explicitly set on each collection.

[heyharvs]

I was using waterline without sails and only had one connection in the config. Even though I had set the dominant flag on one of the models, the error was telling me it wasn't set. There is no error saying that I needed the connection.. just looking to help make the error more clear for the next person, that's all.

[devinivy]

If waterline assigns connections to collections automatically when there's only one connection present, I was not aware of it! I think the fix is to either fail earlier (if waterline does not assign the connection) or allow dominance to work correctly when the connection is not specified explicitly (if waterline does assign the connection). @particlebanana do you know what's expected to work here?

[particlebanana]

@heyharvs can you post a failing example so I can better understand how you are handling connections?