WebDAV: handle 403 during Google OAuth authentication

When sending an access token with insufficient scope (for example,
because the Ubuntu Online Accounts service definition was incomplete,
as documented in FDO #86824), Google responds with a 403 "service
denied" error.

Neon (arguably correctly) treats this as a permanent error and not
as a transient authentication error. Google should better send
a 401 error.

To activate the 401 error handling in SyncEvolution, detect this
special case and turn the general SE_ERROR error into SE_AUTH.

src/backends/webdav/NeonCXX.cpp

@@ -603,6 +603,12 @@ bool Session::checkError(int error, int code, const ne_status *status,
         }
     }
 
+    // Detect 403 returned by Google for a bad access token and treat that like
+    // 401 = NE_AUTH. Neon itself doesn't do that.
+    if (m_authProvider && error == NE_ERROR && code == 403) {
+        error = NE_AUTH;
+    }
+
     switch (error) {
     case NE_OK:
         // request itself completed, but might still have resulted in bad status