[PLT-6847] If a message starts with a / but is not a slash command, put the text back in the input box when it fails to send (#6695)

* If a message starts with a / but is not a slash command, put the text back in the input box when it fails to send

* update string per review

* update per review

i18n/en.json

@@ -429,7 +429,7 @@
   },
   {
     "id": "api.command.execute_command.not_found.app_error",
-    "translation": "Command with a trigger of '{{.Trigger}}' not found"
+    "translation": "Command with a trigger of '{{.Trigger}}' not found. To send a message beginning with \"/\", try adding an empty space at the beginning of the message."
   },
   {
     "id": "api.command.execute_command.save.app_error",