multi: change RegisterAttempt error checking order

Move our more generic terminal check forward so that we only
need to handle a single class of expected errors. This change
is mirrored in our mock, and our reproducing tests are updated
to assert that this move catches both classes of errors we get.

routing/mock_test.go

@@ -325,6 +325,10 @@ func (m *mockControlTower) RegisterAttempt(phash lntypes.Hash,
 	_, settled := m.successful[phash]
 	_, failed := m.failed[phash]
 
+	if settled || failed {
+		return channeldb.ErrPaymentTerminal
+	}
+
 	if settled && !inFlight {
 		return channeldb.ErrPaymentAlreadySucceeded
 	}
@@ -333,10 +337,6 @@ func (m *mockControlTower) RegisterAttempt(phash lntypes.Hash,
 		return channeldb.ErrPaymentAlreadyFailed
 	}
 
-	if settled || failed {
-		return channeldb.ErrPaymentTerminal
-	}
-
 	// Add attempt to payment.
 	p.attempts = append(p.attempts, channeldb.HTLCAttempt{
 		HTLCAttemptInfo: *a,