presence: Make PresencePayload a trait, and implement it on every payload.

src/stanza_error.rs

@@ -9,6 +9,7 @@ use std::collections::BTreeMap;
 
 use minidom::Element;
 
+use presence::PresencePayload;
 use error::Error;
 use jid::Jid;
 use ns;
@@ -212,6 +213,8 @@ pub struct StanzaError {
     pub other: Option<Element>,
 }
 
+impl PresencePayload for StanzaError {}
+
 impl TryFrom<Element> for StanzaError {
     type Err = Error;