use console.error instead of console.log for unexpected errors

src/editor/Editor.tsx

@@ -444,7 +444,7 @@ const Editor: React.VFC = () => {
 
                 // istanbul ignore if: should not be reachable
                 if (!(firstItem instanceof HTMLElement)) {
-                    console.log(`bug: firstItem is not an HTMLElement: ${firstItem}`);
+                    console.error(`bug: firstItem is not an HTMLElement: ${firstItem}`);
                     return;
                 }
 

src/firmware/sagas.ts

@@ -751,7 +751,7 @@ function* handleFlashUsbDfu(action: ReturnType<typeof firmwareFlashUsbDfu>): Gen
                 }
 
                 // not expected
-                console.log(err);
+                console.error(err);
             }),
         );