Show a better 404 page if a user wants to edit an exploration but is not logged in.

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Create a private exploration and add another collaborator to it.
2. Send a link to that collaborator via email.
3. As the other collaborator, click that link.

What is the expected output? What do you see instead?

You'll get to a 404 page. This is not very user-friendly, given that you might 
be (after logging in) a collaborator. For such cases, we might want to show a 
page along the lines of “You have reached the edit page for an exploration, 
but you'll need to log in before you can edit it."

Similarly, if a user wants to edit an exploration and does not have 
permissions, perhaps we should show an 'unauthorized' error rather than a 404.

Original issue reported on code.google.com by s...@seanlip.org on 1 Dec 2014 at 2:53