Switch from golang.org/x/net/websocket to nhooyr.io/websocket and handle NotFound errors (#105)

* Switch from golang.org/x/net/websocket to nhooyr.io/websocket

* Do not attach errors that we can handle to the Gin's context

* Add missing newline to "no credentials specified or found, ..." message

* Fix potential NPE in ChooseUsernameAndPassword()

* Fix type in PortForward() error message in "orchard ssh vm"

* Fix potential NPE in Connections()

* Use header.Set() for consistency's sake for Authorization header

internal/proxy/proxy.go

@@ -39,7 +39,7 @@ func Connections(left net.Conn, right net.Conn) (finalErr error) {
 		recordErr(<-rightErrCh)
 	}
 
-	if strings.Contains(finalErr.Error(), "use of closed network connection") {
+	if finalErr != nil && strings.Contains(finalErr.Error(), "use of closed network connection") {
 		finalErr = nil
 	}