MM-63619: improve Groups API error semantics (#30961)

Instead of 5xx errors, return `http.StatusInvalidRequest` when
adding or deleting invalid user ids from groups.

Fixes: https://mattermost.atlassian.net/browse/MM-63619

server/i18n/en.json

@@ -5370,6 +5370,10 @@
     "id": "app.group.uniqueness_error",
     "translation": "group member already exists"
   },
+  {
+    "id": "app.group.user_not_found",
+    "translation": "Error updating group. User with username \"{{.Username}}\" could not be found."
+  },
   {
     "id": "app.group.username_conflict",
     "translation": "user with username \"{{.Username}}\" already exists."