Resource endpoint error message after archive

[winstan]

When a student visits a resource endpoint after their run has been archived, the error message is:
"ENOENT: no such file or directory, lstat '/output/runs/UUID-deliv/student/somedir/index.html'"

It would be informative for students to add "Note: Your run may have been archived; by default only 7 days of runs are preserved."

[steven-wolfman]

We could also give a brief "how to get a new report" like:

For assessments that are still open: You can get a fresh report from the bot as usual on your latest commit. If you need access to a report on an old commit, try: (1) make a branch that starts at that commit, (2) make a trivial new commit on that branch, and (3) call the bot on that commit.