keybase_daemon_rpc: add TODO about using types for session error
keybase_daemon_rpc: add TODO about using types for session error

Suggested by @akalin-keybase.

Issue: #760

keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client