PyAPI: support retrieving the exception when running a string

- Optionally get the error as a single line.
- Support access the error as an allocated string.
- PyC_ExceptionBuffer_Simple was always printing to the `stdout` while
  PyC_ExceptionBuffer didn't, now either print to the output.

Without this, callers are unable to do anything with the error string.

source/blender/python/generic/py_capi_utils.c

@@ -733,7 +733,6 @@ PyObject *PyC_ExceptionBuffer_Simple(void)
 
   PyErr_Restore(error_type, error_value, error_traceback);
 
-  PyErr_Print();
   PyErr_Clear();
   return string_io_buf;
 }