check return value of bind

is common to forget to run this as root, in which case we would like
to see the permission denied error when it tries to open port 69

pkgs/tufted/tftp.lua

@@ -339,7 +339,8 @@ function tftp:listen(rrq_generator_callback, wrq_generator_callback, hosts, port
     local logfile = logfile or io.stderr
     --listen on all given addresses, default to localhost if not given
     for i, address in pairs((type(hosts) == 'table' and hosts) or (hosts ~= nil and{hosts}) or {'127.0.0.1'}) do
-        socket:bind(address, port)
+       local ok, err = socket:bind(address, port)
+       if not ok then error(err .. " binding to " .. address .. ":" .. port) end
     end
 
     --[[The main event loop does two things: