Handle errSeriesDropped correctly

If metrics_relabel_configs are used to drop metrics, an errSeriesDropped
is returned. This shouldn't be used to return an error at the end of a
append() call.

retrieval/scrape.go

@@ -554,6 +554,7 @@ loop:
 			case storage.ErrNotFound:
 				ok = false
 			case errSeriesDropped:
+				err = nil
 				continue
 			default:
 				break loop
@@ -568,6 +569,7 @@ loop:
 			switch err {
 			case nil:
 			case errSeriesDropped:
+				err = nil
 				continue
 			default:
 				break loop