Improve error message when local DoH is enabled without a certificate

Fixes #1136

dnscrypt-proxy/local-doh.go

@@ -61,6 +61,9 @@ func (handler localDoHHandler) ServeHTTP(writer http.ResponseWriter, request *ht
 
 func (proxy *Proxy) localDoHListener(acceptPc *net.TCPListener) {
 	defer acceptPc.Close()
+	if len(proxy.localDoHCertFile) == 0 || len(proxy.localDoHCertKeyFile) == 0 {
+		dlog.Fatal("A certificate and a key are required to start a local DoH service")
+	}
 	noh2 := make(map[string]func(*http.Server, *tls.Conn, http.Handler))
 	httpServer := &http.Server{
 		ReadTimeout:  proxy.timeout,