Handle failures when initializing SystemLookup

If SystemLookup fails to initialize, clients might get spurious exceptions; these are caused by the fact that `SystemLookup` needs to load some native libraries (e.g. ucrtbase on windows). If that fails, an exception is thrown. It would be better to just return an empty lookup - otherwise clients might not be able to use the linker (even if they don't care about looking up symbols in the standard library).