cert-request supports adding service principals that don't exist.
If add is requested for other principal types, the error message
just says "the principal doesn't exist".
Add a new error type with better error message to explain that 'add'
is not supported for host or user principals.
Fixes: https://fedorahosted.org/freeipa/ticket/5991
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>