Fix error displaying exceptions in some cases

Replacing PyErr_Print with PyErr_Display in [0] caused string errors
not to display because PyErr_Display doesn't normalize the exception.

Normalizing before displaying the error resolves this.

[0]: 6a0f98aeef

source/blender/python/generic/py_capi_utils.cc

@@ -890,6 +890,10 @@ PyObject *PyC_ExceptionBuffer()
   PyObject *error_type, *error_value, *error_traceback;
   PyErr_Fetch(&error_type, &error_value, &error_traceback);
 
+  /* Normalizing is needed because it's possible the error value is a string which
+   * #PyErr_Display will fail to print. */
+  PyErr_NormalizeException(&error_type, &error_value, &error_traceback);
+
   /* `io.StringIO()`. */
   PyObject *string_io = nullptr;
   PyObject *string_io_mod = nullptr;

source/blender/python/generic/py_capi_utils.h

@@ -32,7 +32,8 @@ void PyC_StackSpit(void);
  * Return a string containing the full stack trace.
  *
  * - Only call when `PyErr_Occurred() != 0` .
- * - The exception is left in place (without being modified or cleared).
+ * - The exception is left in place without being manipulated,
+ *   although they will be normalized in order to display them (`PyErr_Print` also does this).
  * - `SystemExit` exceptions will exit (so `sys.exit(..)` works, matching `PyErr_Print` behavior).
  * - The always returns a Python string (unless exiting where the function doesn't return).
  */