If doc building is attempted, but fails, then the package page doesn't indicate that. Ideally it would include a log of the failure.
her-lexer-0.1's error is due to the fact that the documentation only partially builds: there is coloured source (which can be seen by changing "doc/" in the 404ing URIs to "doc/src/"), but no actual Haddock pages. This can be seen by downloading /package/her-lexer-0.1/doc.tar (exact format of this URI is TBD).
Does the shiny new build client also upload build reports? If so, it would be useful to expose build reports from bots using the existing server feature.
This is not a Cabal issue.