Missing blocks: add filter allowing customizing the "missing" message

[jeherve]

What problem does this address?

Some themes bundle patterns including blocks that are provided via plugins, or a site owner may have used a block from a plugin, and then disabled the block in the plugin's options or lost access to the feature. At that point, the site editor can only display a generic error message.

This is nice, but in the cases where the plugin providing the block is still active on the site, it would be nice if there was a way for plugin authors to filter the message to offer a workaround to site owners.

What is your proposed solution?

When the messages were first introduced in #22631, we had a editor.missingEdit filter we could use. Unfortunately, that filter was then removed in #25264, and I don't believe the messages can be filtered anymore.

Would it be possible to add such a filter back?

[Mamaduka]

It's possible to override the core/missing block's edit component using the editor.BlockEdit filter or using the same method as Block Directory.

I know it's a bit more code, but general solutions should be preferred over "one-off" filters.