[WiP] Inconsistent context.state exception handling

[peppelinux]

An Exception handling on missing SATOSA_BASE in context.state[STATE_KEY]["requester"].
Probably also related to #228

If a http request come from an arbitrary link, like

https://disco.example.org/role/idp.ds?entityID=https%3A%2F%2Fproxy.example.org%2FSaml2%2Fmetadata&return=https%3A%2F%2Fexample.org%2FSaml2%2Fdisco

the satosa frontend cannot run SatosaBase._auth_req_callback_func, so it never inizializes state[STATE_KEY] = {"requester": internal_request.requester}.

This PR would introduce a self explanatory error message to user, this permit them to understand the nature of the problem without calling help-desk support.

I admit that this sounds, as it is now, too much hardcoded, I'd like to start a discussion with you to handle in a definitive way this kind of feature.

A good point of discussion could be the possibility to disallow backend's authnRequest if context.state would be inconsistent, I think that this would be a better strategy (also with a human readable message to users that warns on this)

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[peppelinux]

Also related to
#228

From that thread there would be some usefull points of discussion I think