[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",

webapp/components/create_post.jsx

@@ -170,10 +170,11 @@ export default class CreatePost extends React.Component {
                     if (err.sendMessage) {
                         this.sendMessage(post);
                     } else {
-                        const state = {};
+                        this.setState({
-                        state.serverError = err.message;
+                            serverError: err.message,
-                        state.submitting = false;
+                            submitting: false,
-                        this.setState({state});
+                            message: post.message
+                        });
                     }
                 }
             );