tapfreighter: unlock asset coins before err on tx broadcast fail

Ensure locked asset coins are released before returning an error when
attempting to publish the anchor transaction. Without this change, if
the chain bridge logic determines the anchor transaction should not be
broadcast, the coins remain locked, leaving the send attempt stuck in
limbo.

Author: ffranr

tapfreighter/chain_porter.go

@@ -1391,6 +1391,18 @@ func (p *ChainPorter) stateStep(currentPkg sendPackage) (*sendPackage, error) {
 				"transaction %v: %w", txHash, err)
 
 		case err != nil:
+			// If the error is due to the min relay fee not being
+			// met, we'll unlock the inputs we locked for this
+			// transfer before returning the error.
+			if strings.Contains(
+				err.Error(), "min relay fee not met",
+			) {
+				p.unlockInputs(ctx, &currentPkg)
+			}
+
+			// We exercise caution by not unlocking the inputs in
+			// the general error case, in case the transaction was
+			// somehow broadcasted.
 			return nil, fmt.Errorf("unable to broadcast "+
 				"transaction %v: %w", txHash, err)
 		}