Update save task to allow an unspecified size and an opening mode.

The check now avoid creating the file (and overriding it) and emit a warning if asked to override an existing file. (True as result but non empty traceback).
Update the measure system to handle warnings emitted by the task when enqueuing.

Now if the result of the tests is positive but the traceback is not empty, display an ErrorDisplay dialog letting the user proceed or cancel the enqueuing.