Hints on channel not found error

[LilithHafner]

$ julia +lts
Error: The Julia launcher failed to determine the command for the `lts` channel.

Caused by:
    Invalid channel specified

Could hint to try juliaup add lts or, even better, prompt in the way modern Pkg does on using and offer to do it for me :)

[davidanthoff]

Yep, great idea!

In general, we should add some functionality that detects whether we can run interactively, and then do a lot more helpful things if we are.