Minor incoming messages refactor and send m.notice on decryption erro…

…rs (#496)

We were falling through without propagating errors anywhere. I caught
most decryption errors and send them along in the new Err field in
DecryptionResult, to be then passed along to the bridge as a new type of
chat event.

I'm going to do another pass to catch any last decryption errors, but
this should be a big improvement over what we currently do.

pkg/signalmeow/events/message.go

@@ -28,12 +28,13 @@ type SignalEvent interface {
 	isSignalEvent()
 }
 
-func (*ChatEvent) isSignalEvent()   {}
-func (*Receipt) isSignalEvent()     {}
-func (*ReadSelf) isSignalEvent()    {}
-func (*Call) isSignalEvent()        {}
-func (*ContactList) isSignalEvent() {}
-func (*ACIFound) isSignalEvent()    {}
+func (*ChatEvent) isSignalEvent()       {}
+func (*DecryptionError) isSignalEvent() {}
+func (*Receipt) isSignalEvent()         {}
+func (*ReadSelf) isSignalEvent()        {}
+func (*Call) isSignalEvent()            {}
+func (*ContactList) isSignalEvent()     {}
+func (*ACIFound) isSignalEvent()        {}
 
 type MessageInfo struct {
 	Sender uuid.UUID
@@ -47,6 +48,11 @@ type ChatEvent struct {
 	Event signalpb.ChatEventContent
 }
 
+type DecryptionError struct {
+	Sender uuid.UUID
+	Err    error
+}
+
 type Receipt struct {
 	Sender  uuid.UUID
 	Content *signalpb.ReceiptMessage