In a mapping step on the B2B Developer I am using transformers.

Since the order of execution of transformers is not defined, it is not possible to catch an individual transformer error within a single Flow Map Step.

It should be possible to put a Flow Map Step inside Flow Sequences that model TRY/CATCH logic.

Or mapping errors could be put into two classes: fatal - that stop the transaction and non-fatal - that the transformer itself can recover from, therefore, it may be possible to never return an error from a transformer.