- when running with "errors warn;" it will reset the warnings counter
  (if any) when it returns to a good state.
  This re-enables un-suppressed warnings for the next cycle.
  Also reset the warnings counter on end().

ENH: add short-circuit in HashTable::erase(key)

- skip and return false if the table is empty or the key is not found.
  This makes for faster no-op behaviour.