ChangeData: Throw exception when failing to reload change

If a change cannot be reloaded from the database, instead of
returning null and letting subsequent call sites fail with
a null pointer exception, raise an exception with a message
that will help the administrator investigate.

An example of this is when a change was manually deleted from
the changes table, but references to it from other tables such
as patch_comments were not.

Bug: Issue 3871
Change-Id: I4dece22617d34a42eeb48bdce492124125fe9bc7

gerrit-server/src/main/java/com/google/gerrit/server/query/change/ChangeData.java

@@ -561,6 +561,9 @@ public class ChangeData {
 
   public Change reloadChange() throws OrmException {
     change = db.changes().get(legacyId);
+    if (change == null) {
+      throw new OrmException("Unable to load change " + legacyId);
+    }
     return change;
   }