How and when should this feature warn about missing upload adapter?

[Reinmar]

Followup of #569.

Right now, if you enable any image upload functionality without configuring or loading any adapter, there will be no warning on startup. However, an error will be logged if you paste or drop an image and if you try to use the "Insert image" button.

The problem is that the image upload features are enabled by default in all builds. Which means that people may get warnings even though they are not interested in upload at all.

OTOH, silent errors are what developers hate. I can feel how furious I'd be myself when after enabling an UploadImage plugin I'd get no warning or anything when I tried to paste/drop an image. For developers, this is awful. And documentation won't help here – many of the developers will not read it that precisely or at all. Especially that this process is always made of at least two steps – enabling an adapter and configuring it.

Furthermore, there are issues like when should the "Insert image" button log a warning? When added to the toolbar? When used?

Also, a thing to consider is that checking the existence of an adapter in all these features will bloat a code (not in terms of size but complexity). Perhaps it would be wise to add FileLoader#assertLoader() method which would log a standardised warning if needed.

[Reinmar]

Actually, the more I think about this the more I think that an error or at least a warning should be logged when trying to paste an image into the editor when a CKE5 build is used.

Why?

Because this is developer's console. If a developer is testing pasting images there's 99% chance that he/she actually wants this image. And the error does not change anything for a normal user because the user doesn't see the console at all.

[Reinmar]

Now, funny story – guess who was the first lucky guy who's happy that the error is logged? 😄 👈

I made a typo when configuring the CS integration (which isn't simple, so there's a lot of things to think off) and I realised that the config is not set correctly thanks to that warning. Otherwise, I'd just wonder why nothing's happening after I pasted an image ;)

[Reinmar]

This error works fine – we get statistics that people visit its documentation.