readrequires: provide easier to understand error message
NEW:
$ hg id
abort: unknown repository format: requires feature 'foo' (upgrade Mercurial)!
OLD:
$ hg id
abort: requirement 'foo' not supported!
$ hg init t
$ cd t
$ echo a > a
$ hg add a
$ hg commit -m test
$ rm .hg/requires
$ hg tip
abort: index 00changelog.i unknown format 2!
[255]
$ echo indoor-pool > .hg/requires
$ hg tip
abort: unknown repository format: requires feature 'indoor-pool' (upgrade Mercurial)!
[255]