[infra] Consider improving build failure error messages

### Describe the problem

Currently, when I run `dart run dash_site build` and it fails the output looks like this:

```
$ dart run dash_site serve

Building package executable...
Built dash_site:dash_site.
Running jaspr in static rendering mode.
✓ [CLI] Web compilers started. (3.5s)
✗ [CLI] [ERROR] Failed building web assets. There is probably more output above. (0.9s)
[CLI] [ERROR] Could not start dev server due to build errors.
Terminating web compilers...
```

This doesn't provide any help into where investigate.

### Expected fix

The build error should provide more information, even if its high-level. For example, is there a problem did the scss compiler fail? Or is there an error in a Jaspr component? etc. Just that information would go a long way.

### On which platform(s) did you experience this issue?

_No response_

### Additional context

_No response_

Start by reproducing the failure with `dart run dash_site build` or `dart run dash_site serve` and inspect the output around the web compiler failure. Trace how dash_site reports Jaspr and SCSS or other asset-build errors, then make the final message identify the failing stage or component and verify that the output gives actionable investigation context.

Written by the indexing model from the issue text.