Fix dataset update behavior

I found the info messages a little confusing when `data_path(force_update=True)` is called, here's a suggestion to make them clearer. 

When the archive already exists, saying "no" to overwrite it shouldn't raise an exception, so I fixed that too. (But maybe I misunderstood what this was supposed to do).

Also if file is successfully decompressed as gz, no need to try bz2 (and raise a warning).

Also a typo in "decompressing".

What do you guys think?