Avoid SecurityException when category is not found and Subtext is running in Medium trust

[jeremy-jameson]

Although I was unable to repro this issue using the latest version of Subtext (see issue #13), I believe the issue described for Subtext 2.5 in the following blog post still applies when the request URL specifies an invalid category:

https://www.technologytoolbox.com/blog/jjameson/archive/2012/01/23/building-technologytoolbox-com-part-15.aspx

[jeremy-jameson]

There are a few other places in the Subtext solution where the HttpHelper.SetFileNotFoundResponse method is called (and should probably be updated as well to avoid issues when running in Medium trust). However, since there are multiple options for making those changes, I figure I should leave that up to Phil (or at least wait until we discuss the issue).