executables: Reject Lua 5.1 early Prevents attempting to load libraries that may no longer be found and crashing with a traceback. Platforms like Debian where multiple Lua versions can be installed at the same time and 'lua' pointing to one of the installed interpreters via symlinks, there's the possibility that prosody/prosodyctl may be invoked with Lua 5.1, which will no longer have any of the rest of Prosody libraries available to be require(), and thus would immediately fail with an unfriendly traceback. Checking and aborting early with a friendlier message and reference to more information is better. Part of #1600