luci-base: add cbi tsection error msg option

luci-base: add cbi tsection error msg option

In some situation it is usefull to inform the use that this section
could not get delete, because this config is referenced or is in use.

Example pseudo code:

function s.remove(self, sid)
	<check if secation could get deleted>
	if output == nil then
		self.error_msg = translatef("Unable to remove this section")
		return nil
	end
end

Signed-off-by: Florian Eckert <fe@dev.tdt.de>