The field is prefixed by an ID in LDAP (separated from the key data by
a ':'), strip it before returning key data from the backend.

And make HTTP result codes slightly more meaningful.

This allows Dovecot to fail temporarily if the key lookup backend is
unavailable or has issues talking to LDAP.

The dovecot-keylookupd is a dict-proxy dovecot lookup daemon, which
can read public and private keys from the LDAP backend and use them
for Dovecot userdb / passdb lookups.