PLT-5612: Don't show error on invalid search query. (#5596)

Log a warning to the logs, but don't show an error to the user in the
WebUI, just an empty set of query results, when the fulltext search
query they enter generates a syntax error from the database.

i18n/en.json

@@ -5032,8 +5032,8 @@
     "translation": "You cannot update an existing Post"
   },
   {
-    "id": "store.sql_post.search.app_error",
-    "translation": "We encountered an error while searching for posts"
+    "id": "store.sql_post.search.warn",
+    "translation": "Query error searching posts: %v"
   },
   {
     "id": "store.sql_post.update.app_error",