Launch service thread with a function that re-throws if they fail

In Haskell by default if a thread has an exception, the main thread isn't
notified at all. This patch changes service thread launching to re-throw their
exceptions in the main thread, so that their failure is noticed.

src/Control/Concurrent/Local.hs

@@ -0,0 +1,31 @@
+{- This file is part of Vervis.
+ -
+ - Written in 2019 by fr33domlover <fr33domlover@riseup.net>.
+ -
+ - ♡ Copying is an act of love. Please copy, reuse and share.
+ -
+ - The author(s) have dedicated all copyright and related and neighboring
+ - rights to this software to the public domain worldwide. This software is
+ - distributed without any warranty.
+ -
+ - You should have received a copy of the CC0 Public Domain Dedication along
+ - with this software. If not, see
+ - <http://creativecommons.org/publicdomain/zero/1.0/>.
+ -}
+
+module Control.Concurrent.Local
+    ( forkCheck
+    )
+where
+
+import Prelude
+
+import Control.Concurrent
+import Data.Functor (void)
+
+-- | Like 'forkIO', but if the new thread terminates with an exception,
+-- re-throw it in the current thread.
+forkCheck :: IO () -> IO ()
+forkCheck run = do
+    tid <- myThreadId
+    void $ forkFinally run $ either (throwTo tid) (const $ return ())