Distribution/Client/Tar.hs: make clearer error message when tar fails to unpack source

Patch improves error reporting to give a clue what broke
when unpacking:
    $ dist/build/cabal/cabal unpack hackport
    Unpacking to hackport-0.3.2/
    cabal: File in tar archive is not in the expected directory "hackport-0.3.2"
    but got the following hierarchy: [".",".","@LongLink"]

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>