LaTeX reader: Better error messages for environments.

Now it should tell you that it was looking for \end{env},
instead of giving "unknown parse error."

src/Text/Pandoc/Readers/LaTeX.hs

@@ -801,7 +801,9 @@ descItem = do
   return (ils, [bs])
 
 env :: String -> LP a -> LP a
-env name p = p <* (controlSeq "end" *> braced >>= guard . (== name))
+env name p = p <*
+  (try (controlSeq "end" *> braced >>= guard . (== name))
+    <?> ("\\end{" ++ name ++ "}"))
 
 listenv :: String -> LP a -> LP a
 listenv name p = try $ do