Log when Compile and ErrorCheck resources are requested but not available

[patritzenfeld]

Maybe we could just log when one of the semaphores is exhausted, but there's another request waiting, e.g. something like this in place of the plain MSem.with sem action calls that are currently used:

waitAndLogExhausted name sem action = do
  open <- MSem.peekAvail sem
  when (open < 1) $
    hPutStrLn stderr $ name ++ " has exhausted its available resources, but there is further demand." 
  MSem.with sem action

with waitAndLogExhausted "Compiles" (compileSem ctx) someIO

Then we can see how often requests have to queue up because of the current limit and can try increasing the values.

Originally posted by @patritzenfeld in #65 (comment)