* fix(serve): provide a clear error with pathless proxy URLs
Without this, `dx serve` panics with this message:
```
Paths must start with a `/`. Use "/" for
root routes
```
That's not very clear. Instead, we can detect this situation and provide a better error message:
```
Error: 🚫 Serving project failed: Failed to establish proxy: Proxy backend URL must have a non-empty path, e.g. http://localhost:8080/api instead of http://localhost:8080
```
* docs(config): correct format for `web.proxy`
