haddock-error gets buried under warnings

I am using

```
stack build --haddock --haddock-deps --haddock-arguments=--hyperlinked-source
```

This fails, because haddock gives an error on one of the source files (ghc does not). But it is hard to find out from stack's output:
- last lines of stack output are just last lines of haddock warnings (and I have hundreds, like "missing documentation for ...") - I would expect a status message from stack saying that something failed
- so, to check the status, and see the error, I have to scroll back on the screen (or in the log file, whose location was printed by stack, but to see that, I also have to wade back throught the warnings)