Improve pygrub error reporting when opening ext2 fs is not possible

Improve pygrub error reporting when opening ext2 fs is not possible

As reported in the thread:
http://lists.xensource.com/archives/html/xen-users/2006-03/msg00721.html,
pygrub does not open ext2 file systems in partitioned images or sub
partitions on e.g. CentOS/RHEL 4, because e2fsprogs ext2fs_open does not
support an offset into the file to be opened.

With this patch, the error is correctly reported instead of a generic
"unable to open file" (and leaving the user searching in the dark).

Signed-off-by: Michael Paesold <mpaesold@gmx.at>