Invalid etag cache key should raise an error

Because of Kamils note, and given he's right, we have to raise an error if we're using etag keys which are not using the middleware. However, due to probably my errors, this is raising errors already.

To focus that MR, I proposed not to fix it in that MR, but later. We'll do that.

/cc @ayufan

mentioned in merge request !11333 (merged)

mentioned in merge request !12084 (merged)

mentioned in commit f20949a3

closed via merge request !12084 (merged)

mentioned in commit 823b17cf

mentioned in commit a2fd106b

mentioned in commit d365dec3

mentioned in merge request !14785