* lib/Auth/Source/Pica.php ($message): New property. Error message
  settings.
  (__construct): Read error message settings.
  (handleAuthenticationModuleRuntimeError): New function. Handle runtime
  errors in authentication module.
  (login): Call runtime error handler.

* lib/Auth/Source/Pica.php (login): Handle authentication module runtime
  errors.
  (getAuthenticationModule): Declare protected to ease unit test.