Our communications go from SAP -> BC -> Trading Partner, using BC as a real time RFC (as opposed to an Idoc for example). SAP performs a call function “z_function”, which in turn maps to a routing rule on BC which calls a flow service and posts data to our trading partner. SAP functions can handle error handling in 2 ways: 1) pass back up an “exception”, or 2) pass errors back up into as an export variable and check that variable for data. Currently I am trying to figure out how to pass an “exception” back up to SAP from BC.
Hope thats enough info to help you help me. 
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