Do not print the packages which have a selected implementation after solver failure

[NathanReb]

When the solver fail to find a solution we print out the diagnostics message from 0install.

It appears that in some cases, there are entries in there that are printed, even though the package mentioned had a selected implementation, meaning the solver has nothing interesting to report about it.

It pollutes the already hard to handle error message and it would be nice if it could be avoided.

It is likely that this can be fixed upstream in 0install directly but if for some reason we can't, we should still be able to do this locally in opam-monorepo.