Bug#922914: If cannot change default, then at least return an error

[積丹尼 Dan Jacobson]

Package: xdg-utils
Version: 1.1.3-1
File: /usr/bin/xdg-mime

$ xdg-mime default zzzz.desktop text/html; echo $?
0
$ xdg-mime query default text/html
org.gnome.Epiphany.desktop

Or make a sixth exit code to say what happened in this case.

EXIT CODES
       An exit code of 0 indicates success while a non-zero exit code indicates failure. The following failure codes can be returned:

       1
           Error in command line syntax.

       2
           One of the files passed on the command line did not exist.

       3
           A required tool could not be found.

       4
           The action failed.

       5
           No permission to read one of the files passed on the command line.