Make SAVE-RESOURCE on file folders gracefully handle failure to write

Make SAVE-RESOURCE on file folders gracefully handle failure to write
the file.  Previously killing an IMAIL file folder buffer for a
read-only file would create an *error* buffer and fail to kill the
IMAIL buffer!

(Perhaps it would be better to add a new value to FOLDER-SYNC-STATUS,
to indicate that the persistant storage is immutable.  But putting an
error handler here is useful anyway, because the state of the world
may change between the call to FOLDER-SYNC-STATUS and the actual write
to the disk.)