As suggested by Stefan in [1] I prepared a patch that creates a new 
elisp function which makes a default value for command-error-function. 
This makes it possible to use this variable by packages. Patch is 
against r114884.

Please review and comment my first Emacs patch. I couldn't find any 
guide for submitting patches, so I based it on my intuition and 
experience from previous projects, mainly jEdit.

Sometimes it's better to have a ticket in the tracker for a patch for 
easy reference to a discussion from a commit log message. But in gnu 
they say about posting a patch to the list, so I post.

Thanks,
Jarek

[1] http://lists.gnu.org/archive/html/emacs-devel/2013-10/msg00807.html