converger: Update the API for errors and naming

We updated the API and behaviour to make two changes:

1) We remove the log.* stuff, and replace the Fatal calls with straight
calls to panic(). These are meant to be like an assert, and shouldn't
happen unless there is a user error or a bug.

2) We made the !uuid.IsValid() checks return an error instead of causing
a panic. These returns errors instead, and makes the process safer for
users who are okay calling a function that won't have an effect.

etcd.go

@@ -678,6 +678,7 @@ func (obj *EmbdEtcd) CtxError(ctx context.Context, err error) (context.Context,
 // CbLoop is the loop where callback execution is serialized
 func (obj *EmbdEtcd) CbLoop() {
 	cuuid := obj.converger.Register()
+	cuuid.SetName("Etcd: CbLoop")
 	defer cuuid.Unregister()
 	if e := obj.Connect(false); e != nil {
 		return // fatal
@@ -731,6 +732,7 @@ func (obj *EmbdEtcd) CbLoop() {
 // Loop is the main loop where everything is serialized
 func (obj *EmbdEtcd) Loop() {
 	cuuid := obj.converger.Register()
+	cuuid.SetName("Etcd: Loop")
 	defer cuuid.Unregister()
 	if e := obj.Connect(false); e != nil {
 		return // fatal