OPDS: Fix browsing a Calibre content server

[NiLuJe]

• Actually log failures, besides showing the completely unhelpful InfoMessage.
• Handle searches on Calibre.
• Fix content fill on pagination. I have no frickin' clue how the hell this ever worked, as it was using the wrong handler name.

Fix #11968

(Rebase me).

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This change is 

[NiLuJe]

• I have no frickin' clue how the hell this ever worked, as it was using the wrong handler name.

I can only assume there was a point in time where the OPDSBrowser subclass was reinventing the wheel and re-implementing the navigation arrows with a different handler, because I could have sworn this used to work for realz.

[Frenzie]

I think it did, yes.

[NiLuJe]

Note that we don't handle the "last page" link at all, because that would require being able to backfill the gaps between the current content and the tail of the content, and I don't even want to start thinking about that ;p.

So, right now, our last page chevron jumps to the last page of the currently loaded content, and won't request more content (which means you need to page backward and then forward again to get more stuff).