Werk #7224: Backup/Restore: Fix vanishing files terminating a backup

The "omd backup" mechanism recursively backups all site related files of a Check_MK site. When files in a site directory, that is currently being processed vanish, this could lead to failed backups with errors like this:

Site backup failed: Traceback (most recent call last):
File "/omd/versions/1.5.0p8.cee/bin/omd", line 4553, in
command_function(args, command_options)
File "/omd/versions/1.5.0p8.cee/bin/omd", line 3711, in main_backup
backup_site_to_tarfile(fh, tar_mode, options)
File "/omd/versions/1.5.0p8.cee/bin/omd", line 3686, in backup_site_to_tarfile
backup_site_files_to_tarfile(tar, options)
File "/omd/versions/1.5.0p8.cee/bin/omd", line 3556, in backup_site_files_to_tarfile
tar.add(g_sitedir, g_sitename, exclude=filter_files)
File "/omd/versions/1.5.0p8.cee/lib/python2.7/tarfile.py", line 2032, in add
recursive, exclude, filter)
File "/omd/versions/1.5.0p8.cee/lib/python2.7/tarfile.py", line 2032, in add
recursive, exclude, filter)
File "/omd/versions/1.5.0p8.cee/lib/python2.7/tarfile.py", line 2032, in add
recursive, exclude, filter)
File "/omd/versions/1.5.0p8.cee/lib/python2.7/tarfile.py", line 2032, in add
recursive, exclude, filter)
File "/omd/versions/1.5.0p8.cee/lib/python2.7/tarfile.py", line 2032, in add
recursive, exclude, filter)
File "/omd/versions/1.5.0p8.cee/lib/python2.7/tarfile.py", line 2009, in add
tarinfo = self.gettarinfo(name, arcname)
File "/omd/versions/1.5.0p8.cee/lib/python2.7/tarfile.py", line 1881, in gettarinfo
statres = os.lstat(name)
OSError: [Errno 2] No such file or directory: '/omd/sites/xyz/var/pnp4nagios/perfdata/hastenichgesehn/Interface_BG4_2OG_VLAN_421.xml.new'

After this change the backup continues in such a situation excluding the just vanished file.

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