Rephrase two error messages

Awkwardness spotted by Metalhead64 in
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Multimaps-unknown-class-for-service/en_and_MediaWiki:Multimaps-error-incorrect-class-for-service/en

Change-Id: I71d11490b03a5e3868f85755bad7bd053320527a