Fix some warnings, add meaningful errors

[ndrsllwngr]

APPROACH

• PR points at Feature/lenses #25
• renamed board to board_ to fix shadowing
• commented out unused parts of code in Application.hs, it was not in the MongoDB template, but I wanted at first to make use of it. let's see if we have time to include or evaluate it.
• changed undefined to error "msg" to provide meaningful feedback, e.g. in the Example the action Type is wrong

09/Sep/2020:23:01:49 +0200 [Error#yesod-core] Invalid MoveRequest.
CallStack (from HasCallStack):
  error, called at src/Marshalling.hs:92:25 in minesweepskell-0.1.0-8uy8wOlansCFGGguNCw8nf:Marshalling @(yesod-core-1.6.18-1gehdd9uaBvCwfXwrHEsNB:Yesod.Core.Class.Yesod src/Yesod/Core/Class/Yesod.hs:688:5)
PUT /game/freeletics
  Request Body: {"action":"Reasdfasfdveal","coordX":1,"coordY":6}
  Accept: */*
  Status: 500 Internal Server Error 0.008828s

• only two warning: [-Wunused-top-binds] left, see https://gitlab.haskell.org/ghc/ghc/-/issues/12920