Fix error handling in XPC version of invokeCommands. (#37934)

Two issues:

1. If we got an error we did not log it.
2. If we got an error we did not actually propagate it out to the API consumer.

Author: bzbarsky-apple

src/darwin/Framework/CHIP/MTRDevice_XPC.mm

@@ -552,7 +552,11 @@ - (void)invokeCommands:(NSArray<NSArray<MTRCommandWithRequiredResponse *> *> *)c
                              return;
                          }
 
-                         completion(responses, nil);
+                         if (error != nil) {
+                             MTR_LOG_ERROR("%@ got error trying to invokeCommands: %@", self, error);
+                         }
+
+                         completion(responses, error);
                      });
                  }];
     } @catch (NSException * exception) {