Fail rather than do nonsense when external solver returns a bogus answer

OpamClient.preprocess_request has not been performed in that case, and that can
lead the internal heuristic to remove every package unrelated to the request.
It would sure be better to retry cleanly, but for now this is strictly better, and
anyway this shouldn't happen.
Merge pull request #1279 from OCamlPro/no-inconsistent-solver-nonsense

Fail rather than do nonsense when external solver returns a bogus answer