Show friendly error when unable to connect to Prosody

[mwild1]

When the portal fails to connect to Prosody (e.g. at login), it shows an exception:

               GURU MEDITATION                                                                                                                           
               aiohttp.client_exceptions.ClientConnectorError

It would be nice to handle this, and show an explanatory error page instead. The error may be temporary (e.g. while Prosody is still starting up) or it may require admin intervention (e.g. if Prosody failed to start). We can provide a link to Snikket Troubleshooting in case of the latter.