Surprising error/panic when using protoio package with standard Google protobuf package

[MarcoPolo]

See: libp2p/go-libp2p-kad-dht#1015

It may be common for users who are updating a package like go-libp2p-kad-dht, to not also update their usages of go-msgio to use the pbio package rather than the protoio package. We should detect this (somehow) and provide a clear error (and, perhaps, fulfill the intended request with the pbio package).