Better handling of invalid edits

[icculus]

The fix in #243 is merged, so I'm going to close this, but I'll open an issue in ghwikipp where it'll refuse to accept an edit if pandoc throws an error.

(this particular error was a legacy from our import from MoinMoin, as far as I'm aware, and the preview view will be blank if such an edit were attempted, but that whole thing could be more robust...both in refusing to accept the edit and in the preview showing a "this edit has an error!" message.)

Originally posted by @icculus in https://github.com/libsdl-org/sdlwiki/issues/229#issuecomment-1193248812