Glacier: Raise exception on empty archive upload attempt

Currently you get a weird failure in tree_hash() instead:


    Traceback (most recent call last):
      <snip>
      File "/home/pete/src/glacier-cli/boto/glacier/writer.py", line 65, in tree_hash
        return hashes[0]
    IndexError: list index out of range

Amazon does not allow empty archives to be uploaded, so raising an exception specific to this error is correct.

FWIW I've also written a (much uglier) patch for glacier-cli to allow you to fake uploads of empty archives so git-annex works.