Better error message for verification

Verifying this:

```
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

zer0pwn lives in brandon, manitoba, canada
-----BEGIN PGP SIGNATURE-----
Version: Keybase OpenPGP v0.1.12
Comment: https://keybase.io/crypto

wl4EABEKAAYFAlNZXkQACgkQEzc897CULnnTYQD/bMecus80jZdrYgJVjiIPKZ2f
WP9O+3eCTxbaqDSWa/4A/RIJtzPPlEa+roSEUZub1XbldOH8h45PTcd5QsekQ5Cu
=iVsQ
-----END PGP SIGNATURE-----
```

Web interface gives this error: `Error: you cannot process a message that was encrypted for someone else.`

Clearly its not encrypted, but the related keys have `(probably?)` been removed. Error message should be clearer.

Start in the web interface's message-verification flow and locate the source of the error “you cannot process a message that was encrypted for someone else.” Reproduce the supplied PGP signed message, then compare the handling of signed messages with messages encrypted for another recipient and with removed related keys. Done means the verification failure reports the actual cause clearly.

Written by the indexing model from the issue text.