Bug 1107128 - Improve the failed classification save message

This work fixes Bugzilla bug [1107128](https://bugzilla.mozilla.org/show_bug.cgi?id=1107128).

Pretty straightforward here, I've provided context in the bug for the rationale on the wording change.

![errordialogue](https://cloud.githubusercontent.com/assets/3660661/5286269/c1f6a5c8-7af0-11e4-9fd1-717faed09522.jpg)

Tested on Windows:
FF Release **33.1.1**
Chrome Latest Release **39.0.2171.71 m**

Adding @edmorley for review.