feat: big ahh commit

- agent+proto+backend: transfer service status
- agent: fix returning empty message on nonzero exit status
- backend: refactor collector+commander and handlers dependent on them: implement agent accounting via grpc stats handler

agent/internal/commander/impl.go

@@ -47,8 +47,14 @@ func (*CommandExecutor) Execute(command *proto.Command) (fc *proto.FinishedComma
 		_, err := io.Copy(stderrbuf, stderr)
 		return err
 	})
-	if err := cmd.Wait(); err != nil {
-		return nil, err
+	if waitErr := cmd.Wait(); waitErr != nil {
+		var exitErr *exec.ExitError
+		if !errors.As(waitErr, &exitErr) {
+			return nil, waitErr
+		}
+		fc.Status = int32(exitErr.ExitCode())
+	} else {
+		fc.Status = int32(cmd.ProcessState.ExitCode())
 	}
 	if err := eg.Wait(); err != nil {
 		return nil, err