We aer using standard "manage substitution rules" for creating substitution in portal..
we are validating substitute in backend R/3 system in method IF_EX_WF_SUBSTITUTE~MAINTAIN_SUBSTITUTE by providing our custom mesage.
It's successfully showing our custom message under rule activation along with the below mesage.
"Internal error returned by provider: com.sap.netweaver.bc.uwl.substitution.SubstitutionException".
We Want to supress the above meessage i.e only to show the message which we are providing through the above method.