Avoid seg fault when ext2 blocks cannot be read

[Matt Davis]

All,
I submitted a patch earlier.
Unfortunately (I'm used to CVS and SVN) I was expecting git to query
me for a patch message.  After querying me for the Message-ID git went
ahead and sent the patch off.  I am unsure as to the delay between the
mailing list.  So here is the message that should have accompanied the
patch.
If for some reason the patch does not surface, I will re-submit it
(and apply a proper message).

This references Ticket #22
http://parted.alioth.debian.org/cgi-bin/trac.cgi/ticket/22

While this does not fix the problem.  This does throw an exception and
gracefully exit.  The previous exit ended in a seg fault.

This exception occurs when an ext2 block is not read properly.

-Matt