Exception handing on context.request["SAMLResponse"] KeyError

[peppelinux]

A simple PR that fixes an incorrect Exception Handling and a following Error Http Response to users.

[peppelinux]

Hi @c00kiemon5ter, thank you for your attention.
I have collected some common exceptions in SATOSA. I would have more time to investigate, for now I just share the these footprints:

    frontend = self.module_router.frontend_routing(context)
  File "lib/python3.7/site-packages/satosa/routing.py", line 103, in frontend_routing
    target_frontend = context.state[STATE_KEY]
  File "lib/python3.7/collections/__init__.py", line 1025, in __getitem__
    raise KeyError(key)
KeyError: 'ROUTER'

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  File "lib/python3.7/site-packages/satosa/backends/saml2.py", line 350, in authn_response
    if context.state[self.name]["relay_state"] != context.request["RelayState"]:
  File "lib/python3.7/collections/__init__.py", line 1025, in __getitem__
    raise KeyError(key)
KeyError: 'Saml2'

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  File "lib/python3.7/site-packages/satosa/backends/saml2.py", line 324, in authn_response
    if not context.request["SAMLResponse"]:
KeyError: 'SAMLResponse'

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  File "lib/python3.7/site-packages/satosa/frontends/saml2.py", line 192, in _handle_authn_request
    req_info = idp.parse_authn_request(context.request["SAMLRequest"], binding_in)
KeyError: 'SAMLRequest'

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  File "lib/python3.7/site-packages/satosa/base.py", line 186, in _auth_resp_callback_func
    internal_response.requester = context.state[STATE_KEY]["requester"]
  File "lib/python3.7/collections/__init__.py", line 1025, in __getitem__
    raise KeyError(key)
KeyError: 'SATOSA_BASE'

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