Add query data error handling to datasource-http example

[tolzhabayev]

After merging https://github.com/grafana/grafana-plugin-sdk-go/pull/544 we should include query data error handling in datasource examples. Similar to https://github.com/grafana/grafana-plugin-examples/issues/108

An example of a core plugin that needs to improve it's error handling to work properly with 207 responses:

• https://github.com/grafana/grafana/blob/main/public/app/features/plugins/sql/datasource/SqlDatasource.ts#L167
• Currently the "save & test" will be "green" / "ok" even if the database responds with an error https://github.com/grafana/grafana/discussions/48975

[leventebalogh]

@xnyo do you think this is closable? (as https://github.com/grafana/grafana-plugin-examples/pull/119 is merged)