print error and usage on an unrecognized command

[Guiorgy]

E.g.

$ ./vackup impotr mosquitto_config.tar.gz mosquitto_config
Error: 'impotr' is not a recognized command

"Docker Volume Backup". Replicates image management commands for volumes.

export/import copies files between a host tarball and a volume. For making
  volume backups and restores.

save/load copies files between an image and a volume. For when you want to use
  image registries as a way to push/pull volume data.

Usage:

vackup export VOLUME FILE
  Creates a gzip'ed tarball in current directory from a volume

vackup import FILE VOLUME
  Extracts a gzip'ed tarball into a volume

vackup save VOLUME IMAGE
  Copies the volume contents to a busybox image in the /volume-data directory

vackup load IMAGE VOLUME
  Copies /volume-data contents from an image to a volume