castle-search: read Brave again, and report genuine zero-hit searches as such

Brave: #results no longer exists on the live page; fall back to #main/main.
It was reported as 'no results area at all', i.e. a rate limit.

DuckDuckGo: its 'No results found' notice is itself a .result, so an empty
search looked like broken markup. emptyMarker lets the probe say the engine
itself reported no matches, and the error tells the caller to loosen the
query instead of switching engines.

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