A cheesy fix would simply be to wrap it in IGNORE-ERRORS. But it might cause errors in its present form.
i've learned, painfully, that indiscriminate ignore-errors will almost always bite you back (in the form of wasted debugging time), no matter how innocent they look.
This really is more a QL issue than an ASDF one (although it illustrates an issue with wrapping errors, IMO).
while this is true, the implementation of QL requires a reliable way to hook into the internal state of various versions of ASDF (namely, into the situation when ASDF is looking for a system, and not finding it will lead to an error without QL intervention). if i understand it correctly, this is the crux of this issue.