Restore compatibility with stable Emacs 24.2

Compatibility was broken by the following comments which introduced
`user-error` to the code base which is only available from Emacs 24.2.50
onwards.

cb39f2762a5827d692108d10137407e4651d825d
8d2e4c98fbb6a724f220cba6f6296d2b59488611

Thanks for reporting, @prtv :)