Show error when retype/rename annotation is wrong

[frangio]

If the user gets the annotation slightly wrong, it's simply ignored. For example writing /// @custom:oz-retyped-from TheStruct instead of ...-from TheContract.TheStruct.

If there is a retyped/renamed annotation we should enforce that it matches what the previous name or type was.

This may be a new type of layout error, say bad-annotation that we emit if there is an annotation and it doesn't match what we'd expect.