Abstract over ApplicationCallback errors

This simplifies SignalProtocolError at the cost of an extra heap
allocation for other errors. On its own, that probably isn't worth it,
but this lays groundwork for propagating exceptions / errors back up
in the native environment.

rust/protocol/tests/session.rs

@@ -190,12 +190,12 @@ fn test_basic_prekey_v3() -> Result<(), SignalProtocolError> {
 
         let outgoing_message = encrypt(&mut alice_store, &bob_address, original_message).await?;
 
-        assert_eq!(
+        assert!(matches!(
             decrypt(&mut bob_store, &alice_address, &outgoing_message)
                 .await
                 .unwrap_err(),
-            SignalProtocolError::UntrustedIdentity(alice_address.clone())
-        );
+            SignalProtocolError::UntrustedIdentity(a) if a == alice_address
+        ));
 
         assert_eq!(
             bob_store
@@ -553,10 +553,10 @@ fn bad_message_bundle() -> Result<(), SignalProtocolError> {
         let ptext = decrypt(&mut bob_store, &alice_address, &incoming_message).await?;
 
         assert_eq!(String::from_utf8(ptext).unwrap(), original_message);
-        assert_eq!(
+        assert!(matches!(
             bob_store.get_pre_key(pre_key_id, None).await.unwrap_err(),
             SignalProtocolError::InvalidPreKeyId
-        );
+        ));
 
         Ok(())
     })
@@ -703,7 +703,10 @@ fn message_key_limits() -> Result<(), SignalProtocolError> {
         let err = decrypt(&mut bob_store, &alice_address, &inflight[5])
             .await
             .unwrap_err();
-        assert_eq!(err, SignalProtocolError::DuplicatedMessage(2300, 5));
+        assert!(matches!(
+            err,
+            SignalProtocolError::DuplicatedMessage(2300, 5)
+        ));
         Ok(())
     })
 }
@@ -880,14 +883,12 @@ async fn is_session_id_equal(
         .load_session(bob_address, None)
         .await?
         .unwrap()
-        .alice_base_key()
-        .clone()
+        .alice_base_key()?
         == bob_store
             .load_session(alice_address, None)
             .await?
             .unwrap()
-            .alice_base_key()
-            .clone())
+            .alice_base_key()?)
 }
 
 #[test]