Alerting: Improve errors from failed requests in historian app.

Passing an empty body to the query handler would result in the rather unhelpful
response of "EOF", so add some prefixes to errors in useful places.

apps/alerting/historian/pkg/app/notification/notification.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"net/http"
 
 	"github.com/grafana/grafana-app-sdk/app"
@@ -49,7 +50,7 @@ func (n *Notification) QueryHandler(ctx context.Context, writer app.CustomRouteR
 			ErrStatus: metav1.Status{
 				Status:  metav1.StatusFailure,
 				Code:    http.StatusBadRequest,
-				Message: err.Error(),
+				Message: fmt.Sprintf("parse request: %s", err.Error()),
 			}}
 	}
 
@@ -67,7 +68,7 @@ func (n *Notification) QueryHandler(ctx context.Context, writer app.CustomRouteR
 			ErrStatus: metav1.Status{
 				Status:  metav1.StatusFailure,
 				Code:    http.StatusInternalServerError,
-				Message: err.Error(),
+				Message: fmt.Sprintf("notification query: %s", err.Error()),
 			}}
 	}