Below code snippet can be used for throwing error/exception from External rule to Business Proxy..

TCRMExceptionUtils
.throwTCRMException(
new Exception(),
new TCRMInsertException(),
new DWLStatus(),
9L,
WCCICompositesComponentID.ORG_CALCULATION_UTIL_COMPONENT,
TCRMErrorCode.INSERT_RECORD_ERROR,
WCCICompositesErrorReasonCode.ORGCALCULATIONUTIL_NO_RECORD_IN_DUNS_TABLE,
control, this.errHandler);