Do not log server-side errors in non-debug mode

[Muqsit]

Response codes starting with the digit 5 are server-side errors (as opposed to client-side errors) and are not our concern, at least not in production.
They should be ignored instead.

[23:14:07.777] [Server thread/CRITICAL]: muqsit\tebexapi\utils\TebexException: "Expected response code 200, got 502" (EXCEPTION) in "pmsrc/virions/TebexAPI-master/src/muqsit/tebexapi/connection/handler/SimpleTebexConnectionHandler" at li
ne 57
--- Stack trace ---
  #0 plugins/Tebex-master/src/muqsit/tebex/thread/TebexThread(101): muqsit\tebexapi\connection\handler\SimpleTebexConnectionHandler->handle(object muqsit\tebexapi\connection\request\TebexRequestHolder#574313, array[4])
  #1 pmsrc/src/thread/CommonThreadPartsTrait(93): muqsit\tebex\thread\TebexThread->onRun()
  #2 (): pocketmine\thread\Thread->run()
--- End of exception information ---