Bug 1080408 - Pad wording for 'Unknown revision ID' page

This is a supplementary fix to Bugzilla bug [1080408](https://bugzilla.mozilla.org/show_bug.cgi?id=1080408).

In the main fix in the bug to handle unknown revisions, the class referenced in treeherder.css changed on master due to other work. It was then merged in that state. This change re-targets it so we get some semblance of a page border.

Here is the before and after:

![unknownmessagecurrent](https://cloud.githubusercontent.com/assets/3660661/4988206/acebcb7e-692d-11e4-941e-905c7ffedd2e.jpg)

![unknownmessageproposed](https://cloud.githubusercontent.com/assets/3660661/4988209/b14a22a6-692d-11e4-87af-78525e01947c.jpg)

Tested on Windows:
FF Release **33.1**
Chrome Latest Release **38.0.2125.111 m**

Adding @wlach for review.