Shared: Better error message when default data file not found.

Listing the full path can confuse people who are using
`--self-contained`:  they might have intended the file to be
found locally.  So now we just list the data file name.

src/Text/Pandoc/Shared.hs

@@ -583,8 +583,7 @@ readDefaultDataFile :: FilePath -> IO BS.ByteString
 readDefaultDataFile fname =
 #ifdef EMBED_DATA_FILES
   case lookup (makeCanonical fname) dataFiles of
-    Nothing       -> ioError $ userError
-                             $ "Data file `" ++ fname ++ "' does not exist"
+    Nothing       -> err 97 $ "Could not find data file " ++ fname
     Just contents -> return contents
   where makeCanonical = joinPath . transformPathParts . splitDirectories
         transformPathParts = reverse . foldl go []
@@ -592,7 +591,12 @@ readDefaultDataFile fname =
         go (_:as) ".." = as
         go as     x    = x : as
 #else
-  getDataFileName ("data" </> fname) >>= BS.readFile
+  getDataFileName ("data" </> fname) >>= checkExistence >>= BS.readFile
+   where checkExistence fn = do
+           exists <- doesFileExist fn
+           if exists
+              then return fn
+              else err 97 ("Could not find data file " ++ fname)
 #endif
 
 -- | Read file from specified user data directory or, if not found there, from