Cleanup some duplicate error messages, center message headers
Better error messages on 'opam pin' command-line option

Closes #1278. Thanks for reporting !
Fix bug with 'opam show' on locally-pinned packages
Better error message on requests with pinned packages
Store real versions with `switch export` and pinned packages

Partially fixes #1265, but we need to fix that Not_found as well. Would you have a backtrace ?

This should prevent failures, but wont re-import the pinned state of packages. We should add
it without breaking the existing format...
Cleaned up some messages
Fixes to Properly handle re-pinning, pin editing, etc. and reinstallations
Merge pull request #1283 from OCamlPro/pinning-goodness

Fixes a bunch of issues, mainly related to live (re-)pinning of packages