Improve error message for duplicate constraints

[cmeeren]

Product

Hot Chocolate

Is your feature request related to a problem?

I often get error messages like this:

The specified directive `@myDirective` is unique and cannot be added twice. (My.Type)

The solution you'd like

Give me more information to go on, such as which field this applies to:

The specified directive `@myDirective` is unique and cannot be added twice. (My.Type.Property)

(Other better solutions may exist.)

Otherwise, if adding a large object with many properties, I have to spend an unnecessarily large amount of time hunting around to figure out where the error is.

[michaelstaib]

I think the message should also be not repeatable instead of unique :)