Improve user server error message

[ryanlovett]

When there is an internal server error, jupyterhub reports, "Failed to start your server. Please contact admin." This message should be configurable as part of the deployment. For Berkeley we should point people at Piazza.

A GSI suggested,

it might help to add another line that says something to the effect of: "Often this problem is temporary. Wait a few minutes, refresh your browser, and try again. If the problem persists, please contact the admin at {email address}."

He noted that this advice worked for the student who reported the initial 500 server error.