useful snippet for debugging a package that builds but shouldn't

[raingloom]

I used it to run `make localmodconfig` in a linux build, others might
find it useful as well. Should I contribute it to the cookbook or Guix
itself as well?

```
(define (error-before phase pkg)
  (package
   (inherit pkg)
   (arguments
    (substitute-keyword-arguments
        (package-arguments pkg)
      ((#:phases phases)
       `(modify-phases ,phases
          (add-before (quote ,phase) 'error-on-purpose
           (lambda _
             (error "error on purpose")))))))))
```