API: Return 400 on invalid Annotation requests (#32429)

Signed-off-by: bergquist <carl.bergquist@gmail.com>

pkg/services/annotations/annotations.go

@@ -2,11 +2,16 @@ package annotations
 
 import (
 	"context"
+	"errors"
 
 	"github.com/grafana/grafana/pkg/components/simplejson"
 	"github.com/grafana/grafana/pkg/setting"
 )
 
+var (
+	ErrTimerangeMissing = errors.New("missing timerange")
+)
+
 type Repository interface {
 	Save(item *Item) error
 	Update(item *Item) error