This commit has the aim of making template rendering a bit more
robust. It does so by a.) Accepting also strings for helpers that
expect a number, parsing the number if needed, and b.) Ignoring errors
if a template helper fails to render a value and showing an error in
the logs, instead of failing to render the whole template (leading
to no notification being sent).
Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>
