Refine error handler
    
    Add new handler to handle uncaught errors.
    
    Raise werkzeug.exceptions.BadRequest from view instead of ODCS' own.
    This results in badrequest_error is used correctly instead of using
    validationerror_error.
    
    Fixes #89
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>