GH-27059: Rewrite error messages about msg length (#29252)

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/i18n/en.json

@@ -9277,8 +9277,8 @@
     "translation": "Invalid file ids."
   },
   {
-    "id": "model.draft.is_valid.msg.app_error",
-    "translation": "Invalid message."
+    "id": "model.draft.is_valid.message_length.app_error",
+    "translation": "Draft Message property is longer than the maximum permitted length."
   },
   {
     "id": "model.draft.is_valid.priority.app_error",
@@ -9729,8 +9729,8 @@
     "translation": "Invalid Id."
   },
   {
-    "id": "model.post.is_valid.msg.app_error",
-    "translation": "Invalid message."
+    "id": "model.post.is_valid.message_length.app_error",
+    "translation": "Post Message property is longer than the maximum permitted length."
   },
   {
     "id": "model.post.is_valid.original_id.app_error",