Bottleneck doesn't try to create output dir

[fjahr]

There is a bit of an inconsistency between download and find-bottleneck: download will create the output dir if it doesn't exist, so the command in the readme will succeed even im the dump dir does not exist. On the other hand the find-bottleneck example will fail if the bottleneck dir doesn't exist and it will do so at the very end.

Ideally both commands would work consistently, as a quick fix we could also change the readme or do a check at the beginning of find-bottleneck to make sure that it fails early at least.