Show "Browser access type is not supported" message #345 (#346)

src/panel/graph_panel/graph_ctrl.ts

@@ -758,6 +758,9 @@ class GraphCtrl extends MetricsPanelCtrl {
   private async _getDatasourceRequest() {
     if(this._datasourceRequest.type === undefined) {
       const datasource = await this._getDatasourceByName(this.panel.datasource);
+      if(datasource.access !== 'proxy') {
+        throw new Error(`"${datasource.name}" datasource has "Browser" access type but only "Server" is supported`);
+      }
       this._datasourceRequest.type = datasource.type;
     }
     return this._datasourceRequest;