Do not use simple-error superclass in src/conditions.lisp
* src/conditions.lisp (hook-error-mixin): renamed hook-error ->
  hook-error-mixin; changed superclass simple-error -> error
  (no-such-hook): changed superclass name hook-error ->
  hook-error-mixin
  (duplicate-handler): likewise
  (malformed-hook-handler-binding): likewise