Add Fallback auth and fix parsing of unauthorized, if no error is returned

include/mtx/user_interactive.hpp

@@ -142,6 +142,8 @@ struct Terms
 {};
 struct Dummy
 {};
+struct Fallback
+{};
 }
 struct Auth
 {
@@ -156,7 +158,8 @@ struct Auth
                      auth::MSISDN,
                      auth::OAuth2,
                      auth::Terms,
-                     auth::Dummy>
+                     auth::Dummy,
+                     auth::Fallback>
           content;
 };
 void

lib/structs/errors.cpp

@@ -84,7 +84,7 @@ from_string(const std::string &code)
 void
 from_json(const nlohmann::json &obj, Error &error)
 {
-        error.errcode = from_string(obj.at("errcode").get<std::string>());
+        error.errcode = from_string(obj.value("errcode", ""));
         error.error   = obj.value("error", "");
 
         if (obj.contains("session"))

lib/structs/user_interactive.cpp

@@ -118,6 +118,7 @@ to_json(nlohmann::json &obj, const Auth &auth)
                      [&obj](const auth::OAuth2 &) { obj["type"] = auth_types::oauth2; },
                      [&obj](const auth::Terms &) { obj["type"] = auth_types::terms; },
                      [&obj](const auth::Dummy &) { obj["type"] = auth_types::dummy; },
+                     [&obj](const auth::Fallback &) {},
                    },
                    auth.content);
 }

tests/requests.cpp

@@ -223,6 +223,10 @@ TEST(Requests, UserInteractiveAuth)
   "type": "m.login.dummy",
   "session": "<session ID>"
 })"_json);
+        a.content = auth::Fallback{};
+        EXPECT_EQ(nlohmann::json(a), R"({
+  "session": "<session ID>"
+})"_json);
 
         a.content = auth::EmailIdentity{{
           {"<identity server session id>",