fixes go vet warnings for protorpc_test.go

1) go vet complains about protorpc_test.go:

rpc/protorpc/protorpc_test.go:76: no formatting directive in Errorf call
rpc/protorpc/protorpc_test.go:80: no formatting directive in Errorf call
rpc/v2/protorpc/protorpc_test.go:76: no formatting directive in Errorf call

This fix changes every Errorf that does not use any special formatting to a Error call.

2) I removed a comment at v2/protorpc/protorpc_test.go:79 and updated test description. Error code 400 is set explicitly at:

https://github.com/gorilla/rpc/blob/master/v2/server.go#L150

which I assume was intentional change comparing to v1 version (which yield HTTP 500 for this test-case).