Filesystem.Traversion doesn't check if directory listing succeeded

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=3935

Reported by Adam Montague, Sirius Hosting amontague@siriushosting.com

If you create a Filesystem.Traversion object for a directory that contains a directory that you do not have permissions to list (execute), then you get a non-obvious and lengthy error message. I think its assuming that listing of directories succeeds, instead of either skipping directories with no permission to list, or throwing an exception. Here's the error I get if I foreach a Filesystem.Travsion(".") object when I don't have permission to list the ./models directory:

Bad argument 1 to sizeof().
/usr/local/lib/pike/modules/Filesystem.pmod/module.pmod:287:
    /${PIKE_MODULE_PATH}/Filesystem.pmod/module.Traversion()->create("./
models",0)
/usr/local/lib/pike/modules/Filesystem.pmod/module.pmod:226:
    /${PIKE_MODULE_PATH}/Filesystem.pmod/module->Traversion()

/usr/local/lib/pike/modules/Filesystem.pmod/module.pmod:301:
    /${PIKE_MODULE_PATH}/Filesystem.pmod/module.Traversion()->set_current()

/usr/local/lib/pike/modules/Filesystem.pmod/module.pmod:324:
    /${PIKE_MODULE_PATH}/Filesystem.pmod/module.Traversion()->add(1)

/usr/local/lib/pike/modules/Filesystem.pmod/module.pmod:329:
    /${PIKE_MODULE_PATH}/Filesystem.pmod/module.Traversion()->`+=(1)