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.