issue-126: show full error from the datasource if it response with 422 status code (#128)

* issue-126: show fill error from the datasource if it response with 422 status code

* issue-126: fix CHANGELOG.md

Author: dmitryk-dk

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 * FEATURE: Add support for the `$__range` variable in queries.  It will be transformed to the `[time_from, time_to]` in the Unix format. See [this issue](https://github.com/VictoriaMetrics/victorialogs-datasource/issues/112).
 
+* BUGFIX: show the original error message returned from the VictoriaLogs backend. It should help to troubleshoot problems with query or syntax. See [this issue](https://github.com/VictoriaMetrics/victorialogs-datasource/issues/126).
+
 ## v0.8.0
 
 * FEATURE: add support for the `/select/logsql/stats_query` and `/select/logsql/stats_query_range` API calls. This feature helps to build different panels with statistic data. See [this issue](https://github.com/VictoriaMetrics/victorialogs-datasource/issues/61).

pkg/plugin/datasource.go

@@ -220,6 +220,9 @@ func (d *Datasource) datasourceQuery(ctx context.Context, q *Query, isStream boo
 	}
 
 	if resp.StatusCode != http.StatusOK {
+		if resp.StatusCode == http.StatusUnprocessableEntity {
+			return nil, parseErrorResponse(resp.Body)
+		}
 		return nil, fmt.Errorf("got unexpected response status code: %d", resp.StatusCode)
 	}
 

pkg/plugin/response.go

@@ -288,6 +288,25 @@ func parseStatsResponse(reader io.Reader, q *Query) backend.DataResponse {
 	return backend.DataResponse{Frames: frames}
 }
 
+// parseErrorResponse reads data from the reader and returns error
+func parseErrorResponse(reader io.Reader) error {
+	var rs Response
+	if err := json.NewDecoder(reader).Decode(&rs); err != nil {
+		err = fmt.Errorf("failed to decode body response: %w", err)
+		return err
+	}
+
+	if rs.Status == "error" {
+		return fmt.Errorf("error: %s", rs.Error)
+	}
+
+	if rs.Error == "" {
+		return fmt.Errorf("got unexpected error from the datasource")
+	}
+
+	return nil
+}
+
 // labelsToJSON converts labels to json representation
 // data.Labels when converted to JSON keep the fields sorted
 func labelsToJSON(labels data.Labels) (json.RawMessage, error) {
@@ -328,6 +347,7 @@ type Data struct {
 type Response struct {
 	Status string `json:"status"`
 	Data   Data   `json:"data"`
+	Error  string `json:"error"`
 }
 
 // logStats represents response result from the