Better messaging for multiple primary keys

When uploading a public key, the suggested command, "gpg --armor --export you@site.com", will generate multiple keys for some users. If you paste in the result, Keybase says:

"Whoa: Error: cannot have 2 primary keys"

Instead the error should provide guidance on how to fix, e.g.:

"Whoa: Error: cannot have 2 primary keys. To export just one, try `gpg -K; gpg --armor --export ABCD1234'"

No source file or test is named. Start by locating the code that emits the “cannot have 2 primary keys” message and inspect how public-key upload errors are tested. Done means the error explains how to identify one key and export it, with coverage for the multiple-primary-key case.

Written by the indexing model from the issue text.