Do not error when adding an already installed channel.

[GunnarFarneback]

Make it benign to add a channel that was already installed.

Before PR:

julia> run(`juliaup add 1.6.7`)
Error: '1.6.7' is already installed.
ERROR: failed process: Process(`juliaup add 1.6.7`, ProcessExited(1)) [1]

After PR:

julia> run(`juliaup add 1.6.7`)
'1.6.7' is already installed.
Process(`target/debug/juliaup add 1.6.7`, ProcessExited(0))

Fixes #573.

Disclaimer: I have never written a single line of Rust, so anything in this small PR could potentially be wrong.

[albheim]

If it is not an error to add an already added channel it probably is not an error to remove a channel that is not added in src/command_remove.rs either.

I think I agree that error is a bit strong for this, and it should just be information.