Show warnings in UI if query have returned some warnings (#5964)

* Show warnings in UI if query have returned some warnings
+ improve warning (error) text if query to remote was finished with error
* Add prefixes for remote_read errors

Signed-off-by: Stan Putrya <root.vagner@gmail.com>

storage/remote/client.go

@@ -21,6 +21,7 @@ import (
 	"io"
 	"io/ioutil"
 	"net/http"
+	"strings"
 	"time"
 
 	"github.com/gogo/protobuf/proto"
@@ -155,13 +156,14 @@ func (c *Client) Read(ctx context.Context, query *prompb.Query) (*prompb.QueryRe
 		io.Copy(ioutil.Discard, httpResp.Body)
 		httpResp.Body.Close()
 	}()
-	if httpResp.StatusCode/100 != 2 {
-		return nil, errors.Errorf("server returned HTTP status %s", httpResp.Status)
-	}
 
 	compressed, err = ioutil.ReadAll(httpResp.Body)
 	if err != nil {
-		return nil, errors.Wrap(err, "error reading response")
+		return nil, errors.Wrap(err, fmt.Sprintf("error reading response. HTTP status code: %s", httpResp.Status))
+	}
+
+	if httpResp.StatusCode/100 != 2 {
+		return nil, errors.Errorf("remote server %s returned HTTP status %s: %s", c.url.String(), httpResp.Status, strings.TrimSpace(string(compressed)))
 	}
 
 	uncompressed, err := snappy.Decode(nil, compressed)

storage/remote/read.go

@@ -15,6 +15,7 @@ package remote
 
 import (
 	"context"
+	"fmt"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/prometheus/pkg/labels"
@@ -70,7 +71,7 @@ func (q *querier) Select(p *storage.SelectParams, matchers ...*labels.Matcher) (
 
 	res, err := q.client.Read(q.ctx, query)
 	if err != nil {
-		return nil, nil, err
+		return nil, nil, fmt.Errorf("remote_read: %v", err)
 	}
 
 	return FromQueryResult(res), nil, nil

web/ui/static/js/graph/index.js

@@ -513,6 +513,10 @@ Prometheus.Graph.prototype.submitQuery = function() {
           return;
         }
 
+        if ("warnings" in json && json.warnings.length > 0) {
+          self.showWarning(json.warnings.join('<br>'));
+        }
+
         queryHistory.handleHistory(self);
         success(json.data, textStatus);
       },