Fix message for when override conflicts with other directives

[lukassailer]

When working with the override directive i received this error (names changed): @override cannot be used on field "myType.myField" on subgraph "mySubgraph" since "myType.myField" on "mySubgraph" is marked with directive "@requires" But requires was used on "myOtherSubgraph".

I changed the error message to state the correct subgraph and changed the tests to match the expected behavior.

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[lukassailer]

Did i miss something that causes circleci to fail? If so please let me know.