Guard pause/resume against disabled backends; clean up CLI errors

- PauseBackend and ResumeBackend return an error (not bool) when the
  backend is disabled, preventing an inconsistent state where the
  health state says "paused" but enabled=false.
- DisableBackend and EnableBackend now log uniform backend-transition
  lines with from/to instead of separate backend-disable/backend-enable
  messages.
- CLI errors strip gRPC boilerplate ("rpc error: code = ... desc = ")
  and display the server message in red (when color is enabled). Both
  the interactive shell and one-shot mode use the same formatError path.

cmd/maglevc/shell.go

@@ -59,7 +59,7 @@ func runShell(ctx context.Context, client grpcapi.MaglevClient) error {
 			if errors.Is(err, errQuit) {
 				return nil
 			}
-			fmt.Fprintf(rl.Stderr(), "error: %v\n", err)
+			fmt.Fprintf(rl.Stderr(), "%s\n", formatError(err))
 		}
 	}
 }