Issue error with --list-extensions for invalid formats.

Cloess #7797.

src/Text/Pandoc/App/CommandLineOptions.hs

@@ -843,6 +843,12 @@ options =
                              Nothing  -> extensionsFromList extList
                              Just fmt -> getAllExtensions $ T.pack fmt
                      let formatName = maybe "markdown" T.pack arg
+                     if formatName `notElem`
+                         (map fst (readers :: [(Text, Reader PandocPure)]) ++
+                          map fst (writers :: [(Text, Writer PandocPure)]))
+                        then E.throwIO $ PandocOptionError $ formatName <>
+                               " is not a recognized reader or writer format"
+                        else do
                           let defExts = getDefaultExtensions formatName
                           let showExt x =
                                (if extensionEnabled x defExts