protoc-gen-go/grpc: inline errUnimplemented function

[dsnet]

Avoid using an errUnimplemented function, which requires us to
give a name to it, causing possible conflicts.
Instead, just inline it's functionality.

Fixes #817

[solodynamo]

For folks using this repo using go get, their go builds will fail as It would throw errors like imported "google.golang.org/grpc/codes but not used. Its because their .proto files services defination not uses error codes. To fix use the older release of this repo ie. 1.3.1 by compiling using source instead of go get.
In case of docker :
RUN mkdir -p ${GOPATH}/src/github.com/golang/protobuf && \ curl -sSL https://github.com/golang/protobuf/archive/v${PROTOC_GEN_GO_VER}.tar.gz | tar -xz --strip 1 -C ${GOPATH}/src/github.com/golang/protobuf &&\ cd ${GOPATH}/src/github.com/golang/protobuf && \ go build -ldflags '-w -s' -o /golang-protobuf-out/protoc-gen-go ./protoc-gen-go && \ install -Ds /golang-protobuf-out/protoc-gen-go ${OUTDIR}/usr/bin/
where PROTOC_GEN_GO_VER = 1.3.1