Un-typedef errors.ValidationError to buf.validate.Violations

[bufdev]

We're looking to delete buf.validate.Violations in https://github.com/bufbuild/protovalidate/issues/96, and it would also be nice for the Golang API to not hard-depend on a specific version of the Protobuf API. To do so, we can make errors.ValidationError its own type not dependent on the generated Protobuf struct, similar to CompilationError and RuntimeError.