added new 'Error_Messages' postmessage

- so that collbora online can show a custom error msg  with
  nextcloud documentation link

Signed-off-by: Rashesh Padia <rashesh.padia@collabora.com>

src/view/Office.vue

@@ -303,6 +303,7 @@ export default {
 					this.loading = LOADING_STATE.FRAME_READY
 					this.$emit('update:loaded', true)
 					FilesAppIntegration.initAfterReady()
+					this.sendPostMessage('Error_Messages', { websocketconnectionfailed: 'Failed to establish socket connection or socket connection closed unexpectedly. The reverse proxy might be misconfigured, please contact the administrator. For more info on proxy conifguration please checkout https://docs.nextcloud.com/server/latest/admin_manual/office/proxy.html' })
 				} else if (args.Status === 'Document_Loaded') {
 					this.documentReady()
 				} else if (args.Status === 'Failed') {