Try to make the extract operation more robust

[fmaussion]

This is to avoid random test fails like this one: https://travis-ci.org/OGGM/oggm/jobs/476007883

[TimoRoth]

I'm not sure if deleting the target dir is such a good idea. One configuration or code mistake, and it goes on wiping your home or workdir.

[fmaussion]

I'm not sure if deleting the target dir is such a good idea.

We delete dirs all the time in OGGM, since always. This part of the code is pretty robust and hidden anyway.

-> but I understand your concern and it might be worthwhile reviewing all the occurrences of dir delete in OGGM in a separate PR

[fmaussion]

In this case I'm not even sure why this is needed in the first place, the extractall operation really randomly failed with a fileexists error from time to time (also on cluster)