Tolerate inability to write settings file.

Tolerate inability to write settings file.

It's questionable that the unsolicited write of the settings is
reasonable at all, but at the very least it shouldn't crash if it
can't, e.g., when outputting to /dev/null for testing.

TESTED:
Now prints message and continues on failed attempt to write
/dev/null.settings.  Writes settings successfully with real
output file specified.