selected_idps variable now get the only available idp instead of None

otherwise next logger message will print None
Better exception handling on missing IDP configuration. Better LOG message if only one idp is available

	new file:   djangosaml2/exceptions.py
	modified:   djangosaml2/views.py

Author: peppelinux

djangosaml2/exceptions.py

@@ -0,0 +1,2 @@
+class IdPConfigurationMissing(Exception):
+    pass

djangosaml2/views.py

@@ -49,6 +49,7 @@
 
 from djangosaml2.cache import IdentityCache, OutstandingQueriesCache
 from djangosaml2.cache import StateCache
+from djangosaml2.exceptions import IdPConfigurationMissing
 from djangosaml2.conf import get_config
 from djangosaml2.overrides import Saml2Client
 from djangosaml2.signals import post_authenticated
@@ -144,7 +145,13 @@ def login(request,
                 'available_idps': idps.items(),
                 'came_from': came_from,
                 })
-
+    else:
+        # is the first one, otherwise next logger message will print None
+        if not idps:
+            raise IdPConfigurationMissing(('IdP configuration is missing or '
+                                           'its metadata is expired.'))
+        selected_idp = list(idps.keys())[0]
+    
     # choose a binding to try first
     sign_requests = getattr(conf, '_sp_authn_requests_signed', False)
     binding = BINDING_HTTP_POST if sign_requests else BINDING_HTTP_REDIRECT