convert: fix argument mismatch at formatting the abort message
This patch fixes argument mismatch at formatting the abort message,
introduced by
a204fd9b5ba9: the last '%s' doesn't have corresponded
argument.
This patch uses "unexpected size" in the abort message, to distinguish
the reason of failure from "unexpected type" failure checked in the
prior code path below:
if info[1] != type:
raise util.Abort(_('cannot read %r object at %s') % (type, rev))