pivy-box should not fail when PCSC cannot find a reader

[jclulow]

Without a Yubikey connected, and without an SSH agent available, the program exits rather than entering the recovery menu:

$ pivy-box stream decrypt some.ebox >some.plain.txt
pivy-box: 'stream decrypt' command failed
  Caused by PCSCError: SCardListReaders failed: -2146435026 (Cannot find a smart card reader.)
    in piv_find() at piv.c:1268