{{msg?.payload} in markdown breaks the entire dashboard

### Current Behavior

I mistakenly forgot to close the second bracket pair in:

```
{{msg?.payload}
```

And the entire dashboard vanished, without any hint as to where the error might be.

### Expected Behavior

I should get either a runtime error or an error message rendered in the dashboard

### Steps To Reproduce

Create a markdown node and set the contents to

```
{{msg?.payload}
```

Season to taste, deploy, and try to access the dashboard

### Environment

- Dashboard version: 1.8.1
- Node-RED version: 3.1.9
- Node.js version: whatever is shipping in the `node-red` Docker image
- npm version: whatever is shipping in the `node-red` Docker image
- Platform/OS: macOS
- Browser: doesn't matter. Fails on Edge, Safari, Firefox..

### Have you provided an initial effort estimate for this issue?

I am not a FlowFuse team member