schema contracts: more detailed information on why composition failed (tag instead of inaccessible)

Due to the implementation of schema contracts (transforming `@tag` to `@inaccessible`), the composition errors will contain error messages such as "Failed to compose as X is `@inaccessible`". This is correct from the composition point of view. But for the end user, the information that the field is inaccessible due to not being tagged properly is more useful. As he never applied `@inaccessible` on it directly.