Suppress internal errors in client output

Until recently, Gitaly was silently swallowing any errors returned by
SSH `git upload-pack` processes. Clients would still receive stderr
output and a non-zero return code, but Gitlab-Shell would receive error
as nil and log success.

With 9deaf47f1ecb00f0f36d18ee4a0fb1576f5a0efe Gitaly will now return an
error when git fails, but this causes Gitlab-Shell to print out the
GRPC error code as a message to the client:

> fatal: couldn't find remote ref not-a-real-ref
> fatal: the remote end hung up unexpectedly
> remote:
> remote:
> ========================================================================
> remote:
> remote: rpc error: code = Internal desc = SSHUploadPack: exit status 128
> remote:
> remote:
> ========================================================================
> remote:

The `remote:` text gives no additional context for the user and adds
clutter.

This commit suppresses the additional message added by Gitlab-Shell on
failure when the error type is `Internal`, returning client output to
the format it was prior to the Gitaly change.

cmd/gitlab-shell/main.go

@@ -5,6 +5,9 @@ import (
 	"os"
 	"reflect"
 
+	grpccodes "google.golang.org/grpc/codes"
+	grpcstatus "google.golang.org/grpc/status"
+
 	"gitlab.com/gitlab-org/labkit/log"
 
 	shellCmd "gitlab.com/gitlab-org/gitlab-shell/cmd/gitlab-shell/command"
@@ -71,7 +74,9 @@ func main() {
 
 	if err := cmd.Execute(ctx); err != nil {
 		ctxlog.WithError(err).Warn("gitlab-shell: main: command execution failed")
-		console.DisplayWarningMessage(err.Error(), readWriter.ErrOut)
+		if grpcstatus.Convert(err).Code() != grpccodes.Internal {
+			console.DisplayWarningMessage(err.Error(), readWriter.ErrOut)
+		}
 		os.Exit(1)
 	}