New wording when a session cannot save task files

Saving files that come from tasks now goes through a shared set of steps: a check before starting, a check that there is enough disk space, and then opening the file for writing. Destinations are now described as places, and some destinations can report that saving there is always refused.

When a session has nowhere to save files, the message now reads "this session cannot save task files." It used to read "this session keeps no files on this computer."