Show a message when fetching a public key fails

When the server does not return any public keys for a device, it sends a
"public_keys_exhausted" message to the device, which causes it to
generate more.  This change listens for that error, and shows a message
telling the user to try again in a few minutes.

Change-Id: Ib1468eff0754fb28c970e1229435a7ce3208765e