Catch correct exception from shutil.rmtree

Python documentation is not clear on what shutil.rmtree is supposed to
raise. However from the source code, it is obvious that OSError are
raised, but never shutil.Error.

It is not easy to test in normal condition because issues happen
usually in combination with a FUSE filesystem, a probably a race
condition where `fstatat` returns an error just before the filesystem
being unmounted.

Fixes #153.