Check that haddock docs build completed before uploading (fixes #119).

I've created a patch which extends the existing checks in BuildClient.hs:buildPackage to check that the desired-package-name.haddock file exists in the output before uploading docs.tar. The existing check was insufficient and allowed hackage-build to upload docs even when the package has failed to build, provided that at least one dependency succeeded. This fixes issue #119 insofar as hackage-build should stop uploading bad docs.