Not all packages with docs also have build reports about hackage-server

[edsko]

It turns out that this is not true; if there are no build reports, then there are also no docs. However, if there are no no docs, there may or may not be build reports. I did some exhaustive testing on this:

1. For all the cases where there are no build reports, some of the dependencies of the package could not be found and hence it never got to even trying building the package.
2. For the cases where there are build reports, but where we failed to build documentation, we either had errors in the package itself, or the package was an executable, not a library; in the latter case we really shouldn't report the docs as "failed" but rather as "not applicable".

Except for the last issue, for which I've opened #46, this is all perfectly reasonable.

[edsko]

(Incidentally, I thought that the original bug was true, but that was because we had imported docs from the old server, but no build reports.)