backends/saml2: gracefully handle mismatching ACS

[bajnokk]

This is a new attempt to solve #324 . The error message specifically targets the administrator / logs. Even though the exception message has an assumption (the original relay state might be missing for other, more unlikely reasons), but drawing the admin's attention to the mismatching request address and ACS address is a good idea in this case, IMHO.

Commit message follows:

When the IdP redirects to an ACS which has a different address than the one we used for initiating the request, we are unable to verify the RelayState, since the browser does not send the session cookie. In order to make configuration debugging easier, raise an explanatory SATOSAAuthenticationError instead of a KeyError.

While adding a unit test to check for the proper error reporting, some code duplication was refactored.

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[c00kiemon5ter]

closed by d986464 and 62f8775