Add ParseError to PandocExecutionError.

This will be unified with Text.Pandoc.Error eventually. But I'm building
it out here so as not to interfere with other modules that might be
using the error module currently.

src/Text/Pandoc/Class.hs

@@ -112,6 +112,7 @@ getPOSIXTime = utcTimeToPOSIXSeconds <$> getCurrentTime
 -- We can add to this as we go
 data PandocExecutionError = PandocFileReadError FilePath
                           | PandocShouldNeverHappenError String
+                          | PandocParseError String
                           | PandocSomeError String
                           deriving (Show, Typeable)
 
@@ -133,8 +134,9 @@ runIOorExplode ma = do
   eitherVal <- runIO ma
   case eitherVal of
     Right x -> return x
-    Left (PandocFileReadError fp) -> error $ "promple reading " ++ fp
+    Left (PandocFileReadError fp) -> error $ "problem reading " ++ fp
     Left (PandocShouldNeverHappenError s) -> error s
+    Left (PandocParseError s) -> error $ "parse error" ++ s
     Left (PandocSomeError s) -> error s
 
 newtype PandocIO a = PandocIO {