feat: show if error happens while querying (#29)

This makes it a lot easier to debug errors in queries. Previously 
users had to check the response of the query by using the 
browser developer tools.

Now it shows the error in Grafana Panel UI.

Author: Stan van Rooy

src/datasource.ts

@@ -77,7 +77,7 @@ export class DataSource extends DataSourceApi<MyQuery, MyDataSourceOptions> {
             return this.arrayToDataFrame(response.data);
           }),
           catchError((err) => {
-            return of({ data: [] });
+            throw new Error(err.data.message);
           })
         )
       );