now return a single unified InternalBackendError() class,
instead of AssertionError.

This change has a couple of parts:

* assert statements replaced with permanent checks,
  since crypt() is unpredictable enough that we need to have this always on,
  even if production runs code in "-O2" mode.

* added debug_only_repr() helper which allows including sensitive stuff
  like salts & hash digests within error tracebacks -- will only do so
  when global flag is enabled; and that's currently only set by unittest suite.

* added new InternalBackendError() exception class (a RuntimeError subclass);
  which is raised instead of an AssertionError.

--HG--
branch : stable