Check built manpages for errors

Check built manpages for errors

One fix is necessary in gnt-cluster.sgml. Also adding “DELETE_ON_ERROR”
target to remove output file if an error occurred while building it (in
this case the manpage).

This was reported by Iustin Pop in issue 87 and proposed check method
taken from Lintian.
http://code.google.com/p/ganeti/issues/detail?id=87

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>