Documentation improvements and cleanup relating to the IOE monad

Renamed appIOE to tryIOE (it is analogous to 'try' in the standard libraries).
Removed unused IOE operations & documented the remaining ones.
Removed/simplified superfluous uses of IOE operations.

src/compiler/GF/System/Catch.hs

@@ -3,6 +3,7 @@
 module GF.System.Catch where
 import qualified System.IO.Error as S
 
+-- ** Backwards compatible try and catch
 #if MIN_VERSION_base(4,4,0)
 catch = S.catchIOError
 try = S.tryIOError