fix: correct message when root pin failed

this also correctly exits CLI commands with code 1 (was silent
false-positive 0 before)

core/commands/dag/dag.go

@@ -237,7 +237,7 @@ Specification of CAR formats: https://ipld.io/specs/transport/car/
 			}
 
 			if event.Root.PinErrorMsg != "" {
-				event.Root.PinErrorMsg = fmt.Sprintf("FAILED: %s", event.Root.PinErrorMsg)
+				return fmt.Errorf("pinning root %q FAILED: %s", enc.Encode(event.Root.Cid), event.Root.PinErrorMsg)
 			} else {
 				event.Root.PinErrorMsg = "success"
 			}