shop_goods_processing.php: fix phpstan warning
shop_goods_processing.php: fix phpstan warning

> Variable $msg_transaction might not be defined

Throw an exception if the transaction does not match any of the
expected values. This is the simplest way to convince static analysis
that there are no unhandled cases (and it's good future-proofing
anyway).

hemberger/smr,hemberger/smr,smrealms/smr,hemberger/smr,smrealms/smr,hemberger/smr,smrealms/smr