Better error message for `juliaup self channel`

[LilithHafner]

When a user runs juliaup self channel, currently they get an error message that asks them for a channel but does not provide a list of valid/available channels. As the list is and is likely to remain quite short, it would be nice to include in the error message.

x@fedora:~$ juliaup self channel
error: the following required arguments were not provided:
  <CHANNEL>

Usage: juliaup self channel <CHANNEL>

For more information, try '--help'.
x@fedora:~$ juliaup self channel nightly
Error: 'nightly' is not a valid juliaup channel, you can only specify 'release', 'releasepreview' or 'dev'.

[RohitRathore1]

@LilithHafner @davidanthoff

So we want to modify the error message to include the list of valid channels and at the same time ensure this list is displayed both when no channel is provided and when an invalid channel name is used?

If yes then can you please point the correct file path which is having this functionality? I found this file