Refine 404 with contextual data

[jxjj]

Adds a bit more information to the 404 page, per #88.

@cmcfadden - were you thinking something like this? Any edits?

Fixes #88

[cmcfadden]

That's more or less what I was thinking.

Maybe simplify a bit, assuming the audience won't know what Z is?

"The address <z.umn.edu/test> does not exist. Please double check the address, or contact the person who provided you with this link."

[jxjj]

Updated:

on dev