The user experience for error cases is not clearly defined. On the client side, the biggest concern seems to be failure to register -- say, due to a network error when contacting the server. The code currently retries at random intervals in the background, but what do we want the user to see, if any? We can also use this bug to discuss how to display error cases on the web interface if it doesn't make sense to notify the user on the phone.

I experimented with this a bit today. My approach was to, when the registration fails, do: 1) Show the Enable/Disable in the Settings anyway, set to Enable (I would previously wait for the registration to succeed); 2) Show an alert (modal dialog) with the following text: "Something went wrong, and Find My Device could not be activated.\n\nWe will keep trying as long as Find My Device is enabled in the Settings menu."; 3) Keep retrying in the background, unless the user disables the feature in the Settings menu, and without giving any other feedback to the user in case these other registration attempts also fail.

For reference, Bug 945278 added an onerror handler to watch() in Firefox Accounts. If there is an error in getAssertion, it will be sent to this handler. I just found that this worked nicely when my wifi was disabled.

This bug is overly broad. I'm marking it invalid as we'll handle the FxA error case in bug 998489 and filed 1013423 for the unreachable server case.