[ticket/11226] filespec::move_file() should error correctly

PHPBB3-11226
[ticket/11226] Use $user->lang()

PHPBB3-11226
[ticket/11226] Add tests

PHPBB3-11226
[ticket/11226] Explicity set file_moved to false

PHPBB3-11226
Merge pull request #2432 from Nicofuma/ticket/11226

[ticket/11226] filespec::move_file() should error correctly

* Nicofuma/ticket/11226:
  [ticket/11226] Explicity set file_moved to false
  [ticket/11226] Add tests
  [ticket/11226] Use $user->lang()
  [ticket/11226] filespec::move_file() should error correctly