Distinguish between unknown and unavailable packages.
Distinguish between unknown and unavailable packages.

The current error message is rather misleading. For example,

$ opam install ocamlnet.3.5.1
The package "ocamlnet" has no version 3.5.1.
  'opam install ocamlnet.3.5.1' failed

This changeset introduces checks for unavailable packages, yielding
slightly clearer errors:

$ opam install ocamlnet.3.5.1
Version 3.5.1 of "ocamlnet" is incompatible with your compiler.
  'opam install ocamlnet.3.5.1' failed

abate/opam,abate/opam