Do not stop the run when output directory exists

[bouweandela]

Currently, the esmvaltool stops running when the output directory already exists. A common scenario where this happens is if you're running multiple recipes with the same name in parallel, e.g. on a SLURM cluster. This is inconvenient and it would be nice if the tool was able to come up with a good directory name by itself.

[remi-kazeroni]

I like the idea 👍 I've encountered that a couple of times already, precisely when submit several jobs with the same recipe name via SLURM. I think my use case was to test different config files or options. Indeed, it'd be good if the Tool could handle the naming of these output directories and not crash.

[bouweandela]

Since you like the idea, I've asked you for a review in #1818