reflect/protoreflect: adjust ValueOf panic message
reflect/protoreflect: adjust ValueOf panic message

A common mistake is to pass proto.Message to protoreflect.ValueOf
instead of a protoreflect.Message. Specialize for this case
and provide a panic message that is more helpful.

Change-Id: I5def43341aa9607182edde309dea2823e61c1fdb
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/234117
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go