The adapter should catch this exception and send an API error response using the method AuthnApiSupport.writeErrorResponse().

try
{
	...
}
catch (AuthnErrorException e)
{
	// A validation error occurred while processing an API request, return an error response to the API client
	apiSupport.writeErrorResponse(req, resp, e.getValidationError());
	authnAdapterResponse.setAuthnStatus(AUTHN_STATUS.IN_PROGRESS);
	return authnAdapterResponse;
}