RFE: more helpful errcodes

[Peter Sabaini]

It seems `snap install` and `snap refresh` only distinguish between fault/success in their exit codes, returning 1 for different faults; e.g.:

peter@pirx ~/tmp » touch bad.snap
peter@pirx ~/tmp » sudo snap install ./bad.snap --dangerous
error: cannot read snap file: "/var/lib/snapd/snaps/.local-install-328837992" is not a snap or
       snapdir
peter@pirx ~/tmp » echo $?
1
peter@pirx ~/tmp » sudo snap install idontexist
error: snap "idontexist" not found
peter@pirx ~/tmp » echo $?
1
peter@pirx ~/tmp » sudo snap install core --asdfadsf
error: unknown flag `asdfadsf'
peter@pirx ~/tmp » echo $? 1

It would be helpful if the cli returned different error codes for different causes, distinguishing between bad resources, network errors, user errors etc. -- would make it easier when calling `snap` from other code to tailor error handling