Docs: add better error message when generateTranslations.ts fails

scripts/generateTranslations.ts

@@ -567,6 +567,10 @@ function MergeTranslation(source: any, target: any, language: string, context: s
         const sourceV = source[key]
         const targetV = target[keyRemapping?.get(key) ?? key]
 
+        if(targetV === undefined){
+            throw `Merging translations failed; targetV is undefined for context ${context}; but we are trying to add ${sourceV} (${key}) to it`
+        }
+
         if (typeof sourceV === "string") {
             if (sourceV === "") {
                 console.log("Ignoring empty string in the translations")