Hints on channel not found error

```
$ 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 :)

No source files or tests are named in the issue. Start by reproducing the `julia +lts` command and tracing the invalid-channel error path in the CLI. Done means the error provides a useful `juliaup add lts` hint or an agreed interactive way to install the channel.

Written by the indexing model from the issue text.