Comment 3 for bug 1564622

Nate brings up the good point that the command could be a legit Juju 2 command. We're concerned with the case of migrating, but maybe that Juju 1 data is stale and they're wanting to use Juju 2.0. If the user actually types 'juju list-controllers' it doesn't make sense to tell them to run 'juju-1 list-controllers'.

I think the better option is to hint at the juju1, but allow the user to continue the command as is. Example:

    Welcome to Juju 2.0. If you meant to use Juju 1.25 (can we promise/check/make this correct?)
     you can continue using juju-1.25.5 with the command juju-1 e.g. `juju-1 switch`.

    Continue with 'juju $command here' with Juju 2.0? (Y/n)

This would only occur the first time the user ran a command and go away there-after.