Add default value for $numberOfBacktraces in SettledResult::throw

[inxilpro]

#131 changed the signature of SettledResult::throw so that calling ->throw() without a parameter is no longer allowed. This just sets the default to NULL so that it's backwards-compatible.

[daniel-g-wood]

Hey @inxilpro , unfortunately this will still fail in some cases. If you pass null to this function and there are errors, it fails on $this->errorAsString($numberOfBacktraces). errorAsString requires an int parameter, so it throws a type error there instead. You'll also need make the following change (or something like this):

    public function errorAsString(?int $numberOfBacktraces = 2)
    {
        $numberOfBacktraces ??= 2;
        ....
    }