Instead, just go on to the next search path. This prevents a
present-but-unreadable directory from leading to a module load
failure, which happens when one user builds a module and another user
loads it (even if those search paths aren't even being used).
There might still be some other errors that would be useful to treat
as "module found but invalid" rather than "module not found", but
experience has shown that it's the wrong default.
There is a Radar for this but I can't find it.
