Handle missing keys with a default (#3753)

Co-authored-by: Douwe Osinga <douwe@squareup.com>

ui/desktop/openapi.json

@@ -333,8 +333,8 @@
               }
             }
           },
-          "404": {
-            "description": "Configuration key not found"
+          "500": {
+            "description": "Unable to get the configuration value"
           }
         }
       }

ui/desktop/src/api/types.gen.ts

@@ -1015,9 +1015,9 @@ export type ReadConfigData = {
 
 export type ReadConfigErrors = {
     /**
-     * Configuration key not found
+     * Unable to get the configuration value
      */
-    404: unknown;
+    500: unknown;
 };
 
 export type ReadConfigResponses = {