Specify some forms of config validation failures

designs/lightweight-design.md

@@ -182,7 +182,11 @@ From the user's perspective, the functionality of consuming the invite is achiev
 
 Upon scanning the QR code, the software should perform the following actions
 
-1. Validate the decoded contents of the QR code (i.e., to ensure that it contains the expected/necessary grid configuration values and a devicecap), displaying an error message if validation fails
+1. Validate the decoded contents of the QR code, displaying an error message if validation fails -- e.g., because:
+    * The decoded content is in a different format or of a different type than expected
+    * Some necessary configuration value is missing
+    * Some necessary configuration value is in a different format or of a different type than expected
+    * Some additional/unknown configuration value was supplied (which may indicade an out-of-date version of the software)
 2. Store the necessary configuration values decoded from the QR code locally (e.g., in the app-specific storage directory)
 3. Supply the necessary configuration values to the local Tahoe-LAFS client as needed
 4. Use the configured Tahoe-LAFS client to fetch the remote magic-folder contents (e.g., `GET /uri/$devicecap/v1/.magic-folders/` to list magic-folder names, `GET /uri/$devicecap/v1/.magic-folders/$name (personal)` to list magic-folder content)