Improve the error message if no auth set up

[colmjude]

This pull request contains an improved error message for users who have not set up basic auth on their prototypes. It was done after some PaaS testing in which the previous version caused participants a few issues.

There were 2 previous PRs opened for this (See #375 and #377) but this is the tidied up version.

[edwardhorsford]

Thanks for this change @colmjude.

The line of code is getting rather long now - can it be broken up to be more readable?

I feel like the second line could be shortened and front-loaded - something like this:
"You need to set a username and password to make prototypes visible online"

[colmjude]

Hi @edwardhorsford, I made the suggested changes so think this is ready for checking and hopefully merging. Let me know if there is anything else I can do to improve/fix it.

[joelanman]

@colmjude thanks for this, and apologies for the long delay. We now have a team working on the kit.

To aid discussion, the proposed error page looks like this:

the current page looks like this:

[joelanman]

Sorry for the delay, and thanks for your work! Unfortunately this is out of date with the latest code (our fault) so we're going to close it.