luci-base: template.lua: fix nil access in string template error handling
luci-base: template.lua: fix nil access in string template error handling

Template strings have no associated name, which led to an attempt to
concat nil with a string when formatting the error message in case of
exceptions.

Signed-off-by: Jo-Philipp Wich <bd73d35759d75cc215150d1bbc94f1b1078bee01@mein.io>

hnyman/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,rogerpueyo/luci,rogerpueyo/luci,rogerpueyo/luci,rogerpueyo/luci,openwrt/luci,hnyman/luci,lbthomsen/openwrt-luci,openwrt/luci,openwrt/luci,rogerpueyo/luci,hnyman/luci,hnyman/luci,openwrt/luci,hnyman/luci,rogerpueyo/luci,openwrt/luci,openwrt/luci,lbthomsen/openwrt-luci,rogerpueyo/luci,hnyman/luci,hnyman/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,openwrt/luci,hnyman/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,openwrt/luci,rogerpueyo/luci