This commit bubbles the decision of printing models in the binary. Now,
the library do not print models at all.

We have to update the JS worker to dump models as we don't print models
in the library anymore.

In some situations, we need to retrieve the reason of a timeout in the
binary but we have no environment of the SAT solver in these locations.

I add an argument to the timeout exception to save its reason.

This reverts commit dbfed1b.

We use `print_model` twice (in the usual binary and the JS worker), thus
we can factorize it by moving it in `Frontend`.