Provide better error on failed login

[yuvipanda]

We require admins to add each user they wanna permit into the admin
panel. When users who haven't been expressly added, they get a standard
403 Forbidden error.

This page should be a little more informational, and contain information
on why they couldn't log in, and asking them to contact the admins
to get them added. To begin with, I don't think we need to expose who
the admins are, but we can add that optionally later if needed.

[yuvipanda]

We will need to override https://github.com/jupyterhub/jupyterhub/blob/master/share/jupyterhub/templates/error.html in our templates (https://github.com/2i2c-org/pilot-homepage) I think.

[yuvipanda]

@GeorgianaElena this is done, right?

[GeorgianaElena]

@yuvipanda, yeah, this should be fixed by 2i2c-org/default-hub-homepage#2. I forgot to reference this for closing. Closing it now. Thank you!