tapfreighter: use ParSliceErrorCollect for delivering proofs

This commit leverages the new ParSliceErrorCollect function to process
transfer output proofs in parallel. This change ensures that processing
continues even if a transfer output proof delivery instance fails.

tapfreighter/chain_porter.go

@@ -870,11 +870,45 @@ func (p *ChainPorter) transferReceiverProof(pkg *sendPackage) error {
 
 	// If we have a non-interactive proof, then we'll launch several
 	// goroutines to deliver the proof(s) to the receiver(s).
-	err := fn.ParSlice(ctx, pkg.OutboundPkg.Outputs, deliver)
+	instanceErrors, err := fn.ParSliceErrorCollect(
+		ctx, pkg.OutboundPkg.Outputs, deliver,
+	)
 	if err != nil {
 		return fmt.Errorf("error delivering proof(s): %w", err)
 	}
 
+	// If there were any errors during the proof delivery process, we'll
+	// report them here and return the first error we encountered.
+	if len(instanceErrors) > 0 {
+		var firstErr error
+		for idx := range instanceErrors {
+			output := pkg.OutboundPkg.Outputs[idx]
+			instanceErr := instanceErrors[idx]
+
+			// Store the first error encountered.
+			if firstErr == nil {
+				firstErr = instanceErr
+			}
+
+			scriptPubKey :=
+				output.ScriptKey.PubKey.SerializeCompressed()
+			anchorOutpoint := output.Anchor.OutPoint.String()
+			courierAddr := string(output.ProofCourierAddr)
+
+			log.Errorf("Error delivering transfer output proof "+
+				"(anchor_outpoint=%s, script_pub_key=%v, "+
+				"position=%d, proof_courier_addr=%s, "+
+				"proof_delivery_status=%v): %v",
+				anchorOutpoint, scriptPubKey, output.Position,
+				courierAddr, output.ProofDeliveryComplete,
+				instanceErr)
+		}
+
+		if firstErr != nil {
+			return firstErr
+		}
+	}
+
 	// At this point, the transfer is fully finalised and successful:
 	// - The anchoring transaction has been confirmed on-chain.
 	// - The proof(s) have been delivered to the receiver(s).