add/run: improve "already tracked" message

[efiop]

As noted by @ammarasmro

#3561 (comment)

our ERROR: output 'models' is already tracked by SCM (e.g. Git) could be improved.

Looks like we could recommend

git rm -r --cached path
git commit -m "stop tracking path"

as a solution. If the file/dir was accidentally added to git and is really big, then we could also recommend https://rtyley.github.io/bfg-repo-cleaner/ or something like that to remove it from the history.

[efiop]

Let's not recommend bfg for now, as it is too specific.