Graceful handling of missing patches

[Ludovic Courtès]

With commits 6b1f972 and dbab515, patch-not-found errors are now
gracefully handled at run time:

  $ ./pre-inst-env guix build findutils
  guix build: error: WHAT?.patch: patch not found

This introduces an ABI change, so make sure to run:

  make clean-go && make

Thanks,
Ludo’.