[Python] Fix handling of bad input to 'stringify'

Cases where a string is required but a non-string is provided will now generate
an exception instead of substituting the empty string.

interfaces/cython/cantera/_cantera.pxd

@@ -982,7 +982,7 @@ cdef class ReactionPathDiagram:
     cdef CxxStringStream* _log
 
 # free functions
-cdef string stringify(x)
+cdef string stringify(x) except *
 cdef pystr(string x)
 cdef np.ndarray get_species_array(Kinetics kin, kineticsMethod1d method)
 cdef np.ndarray get_reaction_array(Kinetics kin, kineticsMethod1d method)

interfaces/cython/cantera/test/test_thermo.py

@@ -1003,3 +1003,9 @@ class TestQuantity(utilities.CanteraTest):
 
         with self.assertRaises(Exception):
             q1+q2
+
+
+class TestMisc(utilities.CanteraTest):
+    def test_stringify_bad(self):
+        with self.assertRaises(AttributeError):
+            ct.Solution(3)

interfaces/cython/cantera/utils.pyx

@@ -4,7 +4,7 @@ cdef int _pythonMajorVersion = sys.version_info[0]
 cdef CxxPythonLogger* _logger = new CxxPythonLogger()
 CxxSetLogger(_logger)
 
-cdef string stringify(x):
+cdef string stringify(x) except *:
     """ Converts Python strings to std::string. """
     # This method works with both Python 2.x and 3.x.
     if isinstance(x, bytes):