repo: rephrase the "missing requirement" error message
Unknown requirements will now be reported as:
abort: repository requires features unknown to this Mercurial: largefiles!
(see http://mercurial.selenic.com/wiki/MissingRequirement for more information)
Some features of this phrasing:
* avoid double ':' in abort message
* make it more clear who requires and knows what
* don't quote the requirement names - it is not something the user entered or
need the exact spelling of ... and it is "identifiers" that are unambiguous
anyway
* remove double hint by removing "(upgrade Mercurial)" comment
* don't mention upgrading Mercurial without mentioning enabling the feature -
instead, just refer to wiki page for both
* don't just talk about "details", talk about "more information"