Serialize the error as a string to pass it back to the main view

src/renderer/sigFrameScript.js

@@ -12,7 +12,7 @@ window.addEventListener('message', (event) => {
   } catch (error) {
     window.parent.postMessage(JSON.stringify({
       id: data.id,
-      error
+      error: error.toString()
     }), '*')
   }
 })