bip-322: clarify when to return ERROR in verify action

Author: kallewoof

bip-0322.mediawiki

@@ -121,6 +121,8 @@ The resulting signature proof should be encoded using base64 encoding.
 The "Verify" action takes as input a standard flags value, a script sig, an optional witness, and a purpose.
 It emits one of INCONCLUSIVE, VALID, INVALID, or ERROR.
 
+While omitted below, ERROR is returned if an unforeseen error occurs at any point in the process. A concrete example of this is if a legacy proof is given as input to a non-legacy address; the deserialization of the proof will fail in this case, and this should result in an ERROR result.
+
 # Obtain the sighash and scriptPubKey from the purpose; pass on result code if not VALID
 # Verify Script with flags=consensus flags (currently P2SH, DERSIG, NULLDUMMY, CLTV, CSV, WITNESS), scriptSig=script sig, scriptPubKey=scriptPubKey, witness=witness, and sighash=sighash
 # Return INVALID if verification fails