Improve error message on mis-typed extensions.

[blueforesticarus]

Describe your proposed improvement and the problem it solves.
If you try to use a pandoc extension that doesn't exist (ex. inline_note instead of inline_notes), it tells you it "isn't supported for markdown".

$ pandoc -f markdown+inline_note 
The extension inline_note is not supported for markdown

This is highly misleading, since it seems to imply that inline_note is supported for some other format.

This is not good.

1. A user could easily mistype and end up thinking markdown doesn't support inline_notes (or other feature) at all, and give up.
2. A user (me) could assume this was a commonmark vs markdown thing, going down a wild goose chase.

A better alternative would be to check which formats do have the requested extension, and inform the user of those.
If the extension does not exist for any format, the error message should state that it does not exist.

Describe alternatives you've considered.

[tarleb]

The error message now reads

The extension inline_note is not supported for markdown.
Use --list-extensions=markdown to list supported extensions.