when lookup fails, recommend some identifiers that have webfinger resources

[willnorris]

Looking at the access logs for the app, it seems people often try plugging in their Gmail address or a random website URL, see that it gets no result, and then walk away. Instead, we should recommend some values that we know will return a webfinger resource, so they can see a working example.

[pfefferle]

...and/or a link to the https://webfinger.net/code/ page (in case it is a random page).