Python fastdispatch error message improvements

When overloaded C++ methods are called, the fastdispatch mode sometimes
optimises out the generation of the typecheck typemap code, resulting in
the 'Possible C/C++ prototypes' information not being in the error message.
Now when a TypeError is thrown, the exception message is modified to contain the
'Possible C/C++ prototypes'.

See issue #1293

Lib/python/pyerrors.swg

@@ -71,3 +71,33 @@ SWIG_Python_AddErrorMsg(const char* mesg)
     PyErr_SetString(PyExc_RuntimeError, mesg);
   }
 }
+
+SWIGRUNTIME int
+SWIG_Python_TypeErrorOccurred(PyObject *obj)
+{
+  PyObject *error;
+  if (obj)
+    return 0;
+  error = PyErr_Occurred();
+  return error && PyErr_GivenExceptionMatches(error, PyExc_TypeError);
+}
+
+SWIGRUNTIME void
+SWIG_Python_RaiseOrModifyTypeError(const char *message)
+{
+  if (SWIG_Python_TypeErrorOccurred(NULL)) {
+    /* Use existing TypeError to preserve stacktrace and enhance with given message */
+    PyObject *type = NULL, *value = NULL, *traceback = NULL;
+    PyErr_Fetch(&type, &value, &traceback);
+#if PY_VERSION_HEX >= 0x03000000
+    PyObject *newvalue = PyUnicode_FromFormat("%S\nAdditional Information:\n%s", value, message);
+#else
+    PyObject *newvalue = PyString_FromFormat("%s\nAdditional Information:\n%s", PyString_AsString(value), message);
+#endif
+    Py_XDECREF(value);
+    PyErr_Restore(type, newvalue, traceback);
+  } else {
+    /* Raise TypeError using given message */
+    PyErr_SetString(PyExc_TypeError, message);
+  }
+}