SMT backend: better handling of solver returning `unknown`

When the SMT solver returns `unknown`, GRASShopper crashes with
```
Fatal error: exception SmtLibSolver.SmtLib_error(_, "SmtLib: \"line 3086 column 10: model is not available")
```
I guess there should be a more graceful exit.

To reproduce (on the actris branch):
```
./grasshopper.native tests/spl/actris/list.spl -v -procedure thread1 -noinst
```
`-noinst` triggers the `unknown`.