Use error that self-check looks for (#713)

* Use error that self-check looks for

* error

Author: boks1971

engine.go

@@ -248,7 +248,7 @@ func (e *RTCEngine) JoinContext(
 		if verr := e.validate(ctx, url, token, connectParams, ""); verr != nil {
 			return false, verr
 		}
-		return false, err
+		return false, ErrCannotConnectSignal
 	}
 
 	if err = e.waitUntilConnected(); err != nil {
@@ -768,7 +768,7 @@ func (e *RTCEngine) resumeConnection() error {
 		); verr != nil {
 			return verr
 		}
-		return err
+		return ErrCannotConnectSignal
 	}
 
 	e.signalTransport.Start()

errors.go

@@ -30,6 +30,7 @@ var (
 	ErrCannotFindTrack          = errors.New("could not find the track")
 	ErrInvalidMessageType       = signalling.ErrInvalidMessageType
 	ErrInvalidParameter         = signalling.ErrInvalidParameter
+	ErrCannotConnectSignal      = errors.New("could not establish signal connection")
 	ErrNoPeerConnection         = errors.New("peer connection not established")
 	ErrAborted                  = errors.New("operation was aborted")
 )