add link to main application in error messages for private spaces

[valentinRyckaert]

Summary of changes

• add a link to the main instance when you have an error while accessing a private space

Motivation and context

If a user try to access directly a private space without sign in or without enough rights, they get an error and need to change the URL by hand to go to the main application.

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree to license it to the TeSS codebase under the BSD license.