Give more context to rule parser errors. (#4730)

Summary:
- Give record and group name for failing parsing.

pkg/ruler/compat.go

@@ -3,6 +3,7 @@ package ruler
 import (
 	"bytes"
 	"context"
+	"fmt"
 	"io/ioutil"
 	"strings"
 	"time"
@@ -227,7 +228,7 @@ func ValidateGroups(grps ...rulefmt.RuleGroup) (errs []error) {
 		set[g.Name] = struct{}{}
 
 		for _, r := range g.Rules {
-			if err := validateRuleNode(&r); err != nil {
+			if err := validateRuleNode(&r, g.Name); err != nil {
 				errs = append(errs, err)
 			}
 		}
@@ -236,7 +237,7 @@ func ValidateGroups(grps ...rulefmt.RuleGroup) (errs []error) {
 	return errs
 }
 
-func validateRuleNode(r *rulefmt.RuleNode) error {
+func validateRuleNode(r *rulefmt.RuleNode, groupName string) error {
 	if r.Record.Value != "" && r.Alert.Value != "" {
 		return errors.Errorf("only one of 'record' and 'alert' must be set")
 	}
@@ -252,7 +253,7 @@ func validateRuleNode(r *rulefmt.RuleNode) error {
 	if r.Expr.Value == "" {
 		return errors.Errorf("field 'expr' must be set in rule")
 	} else if _, err := logql.ParseExpr(r.Expr.Value); err != nil {
-		return errors.Wrapf(err, "could not parse expression")
+		return errors.Wrapf(err, fmt.Sprintf("could not parse expression for record '%s' in group '%s'", r.Record.Value, groupName))
 	}
 
 	if r.Record.Value != "" {