multi: don't pass err.Error() to Fatalf

It resulted in interpreting the error message as a format string.
Use Fatal(err) instead.

lnwallet/chanfunding/coin_select_test.go

@@ -662,7 +662,7 @@ func TestCoinSelectSubtractFees(t *testing.T) {
 			if err != nil {
 				switch {
 				case test.expectErr == "":
-					t.Fatalf(err.Error())
+					t.Fatal(err)
 
 				case test.expectErr != removeAmounts(err.Error()):
 					t.Fatalf("expected error '%v', got '%v'",