feat(CLI): validate drive (#587)
feat(CLI): validate drive (#587)

The Etcher CLI doesn't care if the drive exists or not. The user will
eventually find out since the CLI will output an `ENOENT` when trying to
actually write data, however its nicer from a UX perspective to catch
this early on.

Change-Type: minor
Changelog-Entry: Validate the existence of the passed drive.
Signed-off-by: Juan Cruz Viotti <8493db2a4f45d66df375afe12f7eba59fe1c8d78@gmail.com>
resin-io/etcher,resin-io/resin-etcher,resin-io/etcher,resin-io/etcher,cedricve/etcher,resin-io/herostratus,resin-io/resin-etcher,cedricve/etcher,resin-io/etcher,resin-io/herostratus,resin-io/etcher,cedricve/etcher