Make a helper function in a C++ test correctly propagate exceptions so that it...

Make a helper function in a C++ test correctly propagate exceptions so that it won't have to spit out compiler warnings.

tests/run/cpp_stl_conversion.pyx

@@ -15,7 +15,7 @@ py_set = set
 py_xrange = xrange
 py_unicode = unicode
 
-cdef string add_strings(string a, string b):
+cdef string add_strings(string a, string b) except *:
     return a + b
 
 def normalize(bytes b):