Haddock errors in dependencies block building of current project

I just encountered the following situation:

```
$ cabal new-build
Build profile: -w ghc-8.2.2 -O1
In order, the following will be built (use -v for more details):
- github-0.21 (lib) (requires build)
- my-project-0.1.0.0 (lib) (configuration changed)
- my-project-0.1.0.0 (exe:my-project) (configuration changed)
Starting github-0.21 (lib)
Building github-0.21 (lib)
Haddock github-0.21 (lib)
Warning: Failed to build documentation for github-0.21 (which is required by exe:my-project from my-project-0.1.0.0).
$ cabal --version
cabal-install version 2.4.0.0
compiled using version 2.4.0.1 of the Cabal library
```

Building with `--disable-documentation` fixed the problem, but required a rebuild of most of my dependencies.

Reproduce the failure with cabal new-build using the reported GHC, Cabal, and github-0.21 versions, then compare it with --disable-documentation. Trace how Haddock failures in dependencies are handled during the build and determine the expected behavior; done should be a documented, tested resolution that avoids an unnecessary dependency rebuild.

Written by the indexing model from the issue text.