fix ErrIndexingInProgress if schema update fails (#6583)

Fixes DGRAPH-2370

If index building fails for some reason after starting building indexes, no new schema mutations
can take place. It throws out errIndexingInProgress error. This is because though the schema
update has failed, the predicate remains in the mutSchema. It should be deleted from the map.

schema/schema.go

@@ -428,6 +428,7 @@ func Load(predicate string) error {
 	if len(predicate) == 0 {
 		return errors.Errorf("Empty predicate")
 	}
+	delete(State().mutSchema, predicate)
 	key := x.SchemaKey(predicate)
 	txn := pstore.NewTransactionAt(1, false)
 	defer txn.Discard()
@@ -448,7 +449,6 @@ func Load(predicate string) error {
 	}
 	State().Set(predicate, &s)
 	State().elog.Printf(logUpdate(&s, predicate))
-	delete(State().mutSchema, predicate)
 	glog.Infoln(logUpdate(&s, predicate))
 	return nil
 }