I don't think there's any harm in testing that the exception message for
a .bz2 file contains the string "unknown archive format". It's unlikely
that we'll want to completely change the error message in future, and if
we do, it will be pretty easy and obvious to fix.


> * unpack_archive raises ValueError when you give an unsupported value to its format
>  parameter, but when it’s in autodetect mode and can’t find an unpacker for the file
>  extension, then it raises a ReadError.  Is it okay to change that to ValueError?

Sounds fine by me.