On https://webkit.org/getting-the-code/ under "OS X or Linux", the step 2 link "snapshot of the WebKit source tree" points to http://nightly.webkit.org/files/WebKit-SVN-source.tar.bz2 which is a 404.
I think I've found the issue. Attempting to build a new source drop now.
I've built a new source drop. I'll have to verify the update mechanism is working properly at the next scheduled cron job runtime.
note: the svn tarball is currently made using svn 1.7. You'll have to run 'svn upgrade' after extracting. I have a work item to update the svn version to 1.9.x.