Improved error handling of misspelled IDPEndpoint ID

[JornWildt]

One often occuring issue we see when installing OIOSAML based applications, is that the ID of an added IDPEndpoint is mispelled - leading to a situation where it cannot find the default IDPEndpoint. The endpoint is created, since it is added manually in the config file, but the metadata file is not found - resulting in an internal NULL pointer reference, which in turn leaves the person installing it without any clue about what is wrong.

Please make a very visible error message for this kind of situation, explaining what is wrong.

We have solved it by adding this check in SAML20FederationConfig:

    internal void Initialize()
        if (MetadataLocation == null)
            return;

        if (!Directory.Exists(MetadataLocation))
            throw new DirectoryNotFoundException(string.Format(Resources.MetadataLocationNotFound, MetadataLocation));

        string[] files = Directory.GetFiles(MetadataLocation, "*.config");
        foreach (string file in files)
        {
            HandleCreateIdp(file);
        }

        // CHECK METADATA FILE
        foreach (var idp in IDPEndPoints)
        {
            if (idp.metadata == null)
              throw new InvalidDataException($"The IDPEndpoint refering to id \"{idp.Id}\" does not have a corresponding metadata file.");
        }