Maintenance: Improve compatibility with GCH 7.8

Define `displayException` for package `base` < 4.8 .

Author: HeinrichApfelmus

haskell/hyper-haskell-server/Main.hs

@@ -1,6 +1,7 @@
 {-----------------------------------------------------------------------------
     HyperHaskell
 ------------------------------------------------------------------------------}
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE GADTSyntax, ExistentialQuantification, RankNTypes, ScopedTypeVariables #-}
 {-# LANGUAGE OverloadedStrings #-}
 module Main where
@@ -126,6 +127,12 @@ toInterpreterError e = case fromException e of
     Just e  -> e
     Nothing -> UnknownError (displayException e)
 
+#if MIN_VERSION_base(4,8,0)
+#else
+displayException :: SomeException -> String
+displayException = show
+#endif
+
 -- | Haskell Interpreter.
 data Hint = Hint
     { run    :: forall a. Interpreter a -> IO (Result a)