Improve error message: `mv: target '/home/runner/.smalltalkCI/_builds/TravisCI.image' is not a directory`

[LinqLover]

This error occurs when running smalltalkCI multiple times after each other in a GitHub Actions job. It might be helpful to introduce a more explicit error message here. :)

[fniephaus]

Instead of throwing an error, smalltalkCI could also print a warning and clean the directory iff running on a CI server so that it can be used multiple times after each other in a single GitHub Actions job. What do you think?

[LinqLover]

Sounds convincing, but I am currently not in the position to overview the entire scope of such a decision. :-)

By the way, I would also discuss this change with respect to #479/#471. Depending on the state of the existing image (downloaded/Trunk updates/project loaded), it might be reused as a cache for the next step. But probably this should be explicitly specified via an extra command line flag if there is any concrete need for such a kind of reuse. :-)