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/Support.hs

@@ -4,6 +4,7 @@ module GF.Support(
            module GF.Infra.Option,
            module GF.Data.Operations,
            module GF.Infra.UseIO,
+           module GF.System.Catch,
            module GF.System.Console,
            -- ** Binary serialisation
            Binary,encode,decode,encodeFile,decodeFile
@@ -13,5 +14,6 @@ import GF.Infra.Location
 import GF.Data.Operations
 import GF.Infra.Option
 import GF.Infra.UseIO
+import GF.System.Catch
 import GF.System.Console
 import Data.Binary