pl.lapp: error message isn't "deterministic", and is misleading

[mooffie]

Run the following program many times, without arguments, under Lua 5.2 or 5.3:

local lapp = require 'pl.lapp'

local args = lapp [[
Arguments:
  <first>  (string)  first argument.
  <second> (string)  second argument.
]]

Sometimes the error message will be:

"missing required parameter: first"

and sometimes:

"missing required parameter: second"

The error message should be the one mentioning "first", not "second".

Under Lua 5.1 (and LuaJIT) the error message is deteministic; it's always:

"missing required parameter: second"

But this, too, should be fixed to mention "first".