Pre-formatted text wrapping change in technical debug page.

In (typically rare) cases, a very long exception line can appear that
requires horiztonal scrolling in the cut-and-paste exception layout box.
To make this fractionally more readable, we pre-wrap the text now.

Fixes #16331, based on a patch from Trinh Hai-Anh.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16637 bcc190cf-cafb-0310-a4f2-bffc1f526a37