Fix incorrect exception handling - seek on closed file

[JacekPliszka]

When exception appeared in etree.XMLSchema after etree.parse(f) is finished - f is already closed and f.seek(0) generates another exception that hides the original one (Python 2.x)

[plq]

I'll accept this with some modifications. I don't remember why the seek is needed but it's best to wrap it in another exception handling block if it's there to prevent it from spoiling the handler.