Make the error encountered when a package can't be found more user-friendly

[bgamari]

Previously we used getPackageDetails to fetch information about the package providing a module. This started failing regularly when GHC moved to package keys, leading to errors of the form haddock: internal error: expectJust getPackageDetails (see haskell/cabal#2297, #363, #361, #353).

Now we use lookupPackage instead, which provides the means for us to gracefully handle failure. The user should now see the slightly more helpful error,

haddock: Unable to find a package providing module "++moduleName pkgMod++ ", skipping Hoogle."

         Perhaps try specifying the desired package explicitly using the --package-name"
         and --package-version arguments."        

Still not great, but hopefully along with a fixed Cabal release this error should occur rather infrequently.

[bgamari]

This works for me.

[Fuuzetsu]

Totally forgot about this. Making some minor changes and pushing it in.

[Fuuzetsu]

Sorry for the hold up. I made some minor changes to the commit (style only) and pushed in.