Rebuild the metadata cache when adding/removing/changing the priority of repositories.
Do not fail badly if opam-lib is not in list of available packages.
Fix both #463 and #446

The error messages are now much clearer:
* in case of a dependency is not available due to compiler/OS constraints
* in case the request deals with non-existing package/version
Merge branch 'master' of github.com:OCamlPro/opam