This is a very targeted addition, which can be generalized. It
makes the following changes:
1. In the solver creation call of the Gurobi interface, i.e.
the constructor `GurobiInterface::GurobiInterface`, the GLOG
severity level is reduced to `DFATAL`, which becomes `ERROR` in
production. This means errors in calls to `GRBloadenv` no longer
automatically kill the process as a side effect.
2. An `std::runtime_error` exception is raised with the return code
and the error message from GRBloadenv. This replaces the functionality
of exiting the process when this error is unhandled. Additionally,
it enables alternate handling by the caller.
3. In the linear solver SWIG wrapper, exception wrapping is turned
on specifically for the `MPSolver::MPSolver` constructor call and
the `std::runtime_error` that may now occur there. This exception
is re-raised as the corresponding RuntimeError in the target language.
Addresses issue #699.
