Make the stack message match the failed validation message
Make the stack message match the failed validation message

* There was a discrepancy between the message in the `Error` object and
  the `stack` property of the `Error` message.  This commit fixes that
  so that those using the default error handlers of most servers see the
  proper message.  _(Note: The error message was proper but the error
  message in the stack was not.)_

Fixes #175

Schoonology/swagger-tools,ddm/swagger-tools,smoebody/swagger-tools,edu2004eu/swagger-tools,MaxKramnik/swagger-tools,schatt/swagger-tools,schatt/swagger-tools,edu2004eu/swagger-tools,amirbaer/swagger-tools,apigee-127/swagger-tools,MaxKramnik/swagger-tools,apigee-127/swagger-tools,ddm/swagger-tools,connected-rcheung/swagger-tools,connected-rcheung/swagger-tools,Schoonology/swagger-tools,Blechhirn/swagger-tools,roshanpaiva/swagger-tools,smoebody/swagger-tools,roshanpaiva/swagger-tools,Blechhirn/swagger-tools,amirbaer/swagger-tools