immutable: raise LayoutInvalid instead of struct.error when a share is truncated

immutable: raise LayoutInvalid instead of struct.error when a share is truncated
To fix this error from the Windows buildslave:

[ERROR]: allmydata.test.test_immutable.Test.test_download_from_only_3_remaining_shares

Traceback (most recent call last):
  File "C:\Documents and Settings\buildslave\windows-native-tahoe\windows\build\src\allmydata\immutable\download.py", line 135, in _bad
    raise NotEnoughSharesError("ran out of peers, last error was %s" % (f,))
allmydata.interfaces.NotEnoughSharesError: ran out of peers, last error was [Failure instance: Traceback: <class 'struct.error'>: unpack requires a string argument of length 4
c:\documents and settings\buildslave\windows-native-tahoe\windows\build\support\lib\site-packages\foolscap-0.3.2-py2.5.egg\foolscap\call.py:667:_done
c:\documents and settings\buildslave\windows-native-tahoe\windows\build\support\lib\site-packages\foolscap-0.3.2-py2.5.egg\foolscap\call.py:53:complete
c:\Python25\lib\site-packages\twisted\internet\defer.py:239:callback
c:\Python25\lib\site-packages\twisted\internet\defer.py:304:_startRunCallbacks
--- <exception caught here> ---
c:\Python25\lib\site-packages\twisted\internet\defer.py:317:_runCallbacks
C:\Documents and Settings\buildslave\windows-native-tahoe\windows\build\src\allmydata\immutable\layout.py:374:_got_length
C:\Python25\lib\struct.py:87:unpack
]
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