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 <jo@mein.io>

modules/luci-base/luasrc/template.lua

@@ -71,7 +71,7 @@ function Template.__init__(self, name, template)
 
 		-- If we have no valid template throw error, otherwise cache the template
 		if not self.template then
-			error("Failed to load template '" .. name .. "'.\n" ..
+			error("Failed to load template '" .. self.name .. "'.\n" ..
 			      "Error while parsing template '" .. sourcefile .. "':\n" ..
 			      (err or "Unknown syntax error"))
 		elseif name then