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/color.go

@@ -2,8 +2,11 @@
 
 package main
 
+import "strings"
+
 const (
 	ansiBlue  = "\x1b[34m"
+	ansiRed   = "\x1b[31m"
 	ansiReset = "\x1b[0m"
 )
 
@@ -20,3 +23,19 @@ func label(s string) string {
 	}
 	return ansiBlue + s + ansiReset
 }
+
+// formatError returns a user-friendly error string. gRPC status errors are
+// unwrapped to show only the server's message (no "rpc error: code = ..."
+// boilerplate). The result is wrapped in red ANSI when color is enabled.
+func formatError(err error) string {
+	msg := err.Error()
+	// google.golang.org/grpc/status errors format as:
+	//   rpc error: code = <Code> desc = <message>
+	if i := strings.Index(msg, " desc = "); i >= 0 {
+		msg = msg[i+len(" desc = "):]
+	}
+	if colorEnabled {
+		return ansiRed + msg + ansiReset
+	}
+	return msg
+}

cmd/maglevc/main.go

@@ -18,7 +18,7 @@ import (
 
 func main() {
 	if err := run(); err != nil {
-		fmt.Fprintf(os.Stderr, "error: %v\n", err)
+		fmt.Fprintf(os.Stderr, "%s\n", formatError(err))
 		os.Exit(1)
 	}
 }

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))
 		}
 	}
 }