The 404 error page renders literal {errorText}

### Expected behavior:
The 404 result presents the actual internal error text in development environments

### Actual behavior:
Literal `{errorText}` is rendered which is a bit less than useful for debugging purposes.

### Steps to fix the problem:
In `async responseAsError(errorText, statusCode)` replace
```

{{errorText}}

```
with
```

${errorText}

```
or similar.