Improve error message

Make error explicitly singular (rather than singular/plural mixture).
Affects:
* revcs-error-unexpected

First spotted at https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Revcs-error-unexpected/en

Change-Id: I7258bacc0567e844cfb0921f2cb825a07530f7e1
Merge "Improve error message"