bug#18907: 25.0.50; long pause when auto-save fails

[Leo Liu]

Sometimes I temporarily modify readonly file buffers and get this error
from auto-save:

        Auto-saving system.el: Opening output file: permission denied, 
/usr/local/src/#system.el#

All good except when it happens it freezes emacs for a few seconds.

Any idea how to make it less intrusive? or make the freeze shorter
namely 0s?

Leo