introduce new RequirementError (issue2649) This improves the misleading error message $ hg identify abort: there is no Mercurial repository here (.hg not found)! to the more explicit $ hg identify abort: requirement 'fake' not supported! for all commands in commands.optionalrepo, which includes the identify and serve commands in particular. This is for the case when a new entry in .hg/requires will be defined in a future Mercurial release.