[Python] Fix exception handling when setting soret_enabled

The C++ exception is now correctly translated to a Python exception, instead of
causing the program to abort.

interfaces/cython/cantera/_cantera.pxd

@@ -671,7 +671,7 @@ cdef extern from "cantera/oneD/StFlow.h":
         void solveEnergyEqn()
         void fixTemperature()
         cbool doEnergy(size_t)
-        void enableSoret(cbool)
+        void enableSoret(cbool) except +
         cbool withSoret()
 
     cdef cppclass CxxFreeFlame "Cantera::FreeFlame":

interfaces/cython/cantera/test/test_onedim.py

@@ -276,6 +276,14 @@ class TestFreeFlame(utilities.CanteraTest):
         self.assertNear(Su_multi, Su_soret, 2e-1)
         self.assertNotEqual(Su_multi, Su_soret)
 
+    def test_soret_flag(self):
+        self.create_sim(101325, 300, 'H2:1.0, O2:1.0')
+        self.assertFalse(self.sim.soret_enabled)
+        with self.assertRaises(RuntimeError):
+            self.sim.soret_enabled = True
+        self.sim.transport_model = 'Multi'
+        self.sim.soret_enabled = True
+
     def test_prune(self):
         reactants= 'H2:1.1, O2:1, AR:5'
         p = ct.one_atm