Merge pull request #133 from erikh/better-errors

Better errors
new assertInterface call; integration into getInterface

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Added a create flag to getInterface in preparation for bigger
modifications.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Several refactors to keep getInterface from always creating a new device

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)