added new 'Error_Messages' postmessage

- handle App_LoadingStatus Initialized
- using Error_Messages richdocuments can pass custom error msg to
  collabora with nextcloud documentation link

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

Author: Rash419

src/view/Office.vue

@@ -222,6 +222,7 @@ export default {
 			})
 		}
 		this.postMessage.registerPostMessageHandler(this.postMessageHandler)
+		this.sendPostMessage('Error_Messages', { websocketconnectionfailed: t('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 configuration please checkout https://docs.nextcloud.com/server/latest/admin_manual/office/proxy.html') })
 
 		this.load()
 	},
@@ -308,6 +309,14 @@ export default {
 				} else if (args.Status === 'Failed') {
 					this.loading = LOADING_STATE.FAILED
 					this.$emit('update:loaded', true)
+				} else if (args.Status === 'Initialized') {
+					// collabora iframe is ready to handle postMessages
+					this.sendPostMessage('Error_Messages', {
+						websocketconnectionfailed: {
+							msg: t('richdocuments', 'Failed to load {productName} - socket connection closed unexpectedly. The reverse proxy might be misconfigured, please contact the administrator. For more info on proxy configuration please checkout', { productName: loadState('richdocuments', 'productName', 'Nextcloud Office') }) + ' %url',
+							url: 'https://docs.nextcloud.com/server/latest/admin_manual/office/proxy.html',
+						},
+					})
 				}
 				break
 			case 'Action_Load_Resp':