We use error codes in standard php exceptions. Those error codes must
be integers. Since php doesn't have enum, we make al list here

0 unrecoverable sql error (connection/database selection)
1 sql error (some must be catched, like duplicate inserts and such)
2 internal error (fout veroorzaakt door programmeerfout of gebruiker die
  scripts omzeilt) permission denied valt hieronder

These faults indicate some serious problems with the system or the user.
Normal errors (login errors occur at the highest level in de code, so they don't
need exceptions).