Fully deprecates msfupdate over SVN

Once this lands, SVN users will no longer be able to update over SVN. Please don't merge this unless and until you want to permanently break msfupdate for SVN users!

After landing this, the msfupdate command for SVN checkouts will exit with code 0x11. These users can still sneak around it by issuing a 'svn update' in the correct svn checkout, but users with old Windows installations who are used to picking "Update Framework" from the start menu will simply see the error and exit out.

# Verification

- [x] Have an svn checkout. This is usually a painful experience (this is why SVN is going away), so it might be best to just use the SVN checkout from Metasploit 4.3 or earlier.
- [x] Replace `msfupdate` with this one
- [x] Run `msfupdate` on the command line, and see that you get a warning and an exit.
- [x] Run "Update Framework" from the Windows start menu (if using 4.3 or earlier), see that you get the message, a pause, then exits.