Notify user when a tool needed for a writer is not installed

[jcotton42]

I've partially done this, but I don't know how to handle the notifications when a tool isn't present, Kindle's custom detection action throws a wrench in it.

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Right now it'll just fail when it tries to call.

A message like "You need wkhtmltopdf to make PDFs, download it at <link>" would be good.

For the GUI, a popup at the start might also be good.

Best to attach this info (and method) to OutputFormat https://docs.python.org/3/library/enum.html#when-to-use-new-vs-init

Wouldn't hurt to put the suffix there either, so it doesn't have to match the enum name.

• CLI
• GUI