Counter the API errors for a single tower

- Sometimes the tower shows up in the main api tower listing but there
  could be intermittent failures in getting data for towers that do
  exist in-game.  In this case at least make it possible to indicate
  that the tower at hand may be temporarily unavailable via EVE API,
  rather than outright removing it like a fully removed/destroyed tower
  as it would have previously.
- This naturally means that towers will no longer be considered gone if
  it shows up in the tower list (which could also be lying due to caches
  or whatever, but at least those missing towers can be indicated).
- Indication is not done in any frontends as the return values have just
  changed here.
Incorporate the use of the error counter.

- Updated the dummy to generate some test cases.