sessionservice: Introduce a well know "not exists" to sessionservice

[rakyll]

Most of the methods in the Service interface can fail if the session doesn't already exist:

type Service interface {
	Get(context.Context, *GetRequest) (StoredSession, error)
	Delete(context.Context, *DeleteRequest) error
	AppendEvent(context.Context, StoredSession, *session.Event) error
}

Introduce a well known error from the package, e.g.ErrSessionNotFound, to let the clients handle this case easily.

[FamALouiz]

This has been open for a while now compared to the other tickets. Would it be possible if I could pick it up and close it? 😄

[rishabhjain1712]

@rishabhjain1712