HTTP Redirect page on UnknowError Exception

[peppelinux]

This PR introduces a new proxy_config.yml parameter called UNKNOW_ERROR_REDIRECT_PAGE, a possibility to redirect to that page all the Users that gets UnknowError Exception in the Http Response.

• Added a UnknownSystemEntity message for humans in satosa.base (We should also use a settings_messages.py file and import them all from there if you agree)
• Added exceptions.SATOSAUnknownErrorRedirectUrl

• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
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• [x] Have you added information on what your changes do and why you chose this as your solution?
• [ ] Have you written new tests for your changes?
• [x] Does your submission pass tests?
• [x] This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?

[peppelinux]

this is another place where a redirect page could be useful

https://github.com/IdentityPython/SATOSA/blob/38dd8a3f9ea5ee8f3e207dcfe910e443d595fb22/src/satosa/proxy_server.py#L98