Skip exception for unset verbosity, use fallback for default solvers

boost::property_try::get without a defaults throws an exception. As
result the nonlinear solver and timestepper think that there is a
problem with solving and chop the timnestep until they give up.

This commit fixes this by using a default and falling back to the
command line specified value for the default solvers.

opm/simulators/linalg/ISTLSolverEbos.hpp

@@ -486,7 +486,7 @@ protected:
         }
 
         bool solve(Vector& x) {
-            const int verbosity = prm_.get<int>("verbosity");
+            const int verbosity = useFlexible_ ? prm_.get<int>("verbosity", 0) : parameters_.linear_solver_verbosity_;
             const bool write_matrix = verbosity > 10;
             // Solve system.