[PATCH 0 of 6] Improve requirement error message

[Pierre-Yves David]

This series propose several improvements to the error message raise by missing
requirements.

I expect the final message to be discussed but there are three major points I'm
trying to improve:

1) provide a link to online help
2) make it clear that the issue is caused by: reading an *on disk* repository
   with a too old *version* of mercurial.
3) list all missing requirement

each point are detailed in commit message