Fix for marking reverse deps of failed packages

The reverse deps may have already failed due to depending on another
failed package. So do not assume all reverse deps are in the
configured state. If they are in the failed state then ignore them.

cabal-install/Hackage/InstallPlan.hs

@@ -234,8 +234,8 @@ failed pkgid buildResult buildResult' plan = assert (invariant plan') plan'
     failures = PackageIndex.fromList
              $ Failed pkg buildResult
              : [ Failed pkg' buildResult'
-               | pkgid' <- packagesThatDependOn plan pkgid
-               , let pkg' = lookupConfiguredPackage plan pkgid' ]
+               | Just pkg' <- map (lookupConfiguredPackage' plan)
+                            $ packagesThatDependOn plan pkgid ]
 
 -- | lookup the reachable packages in the reverse dependency graph
 --
@@ -252,8 +252,17 @@ lookupConfiguredPackage :: InstallPlan a
 lookupConfiguredPackage plan pkgid =
   case PackageIndex.lookupPackageId (planIndex plan) pkgid of
     Just (Configured pkg) -> pkg
-    Just _  -> error $ "InstallPlan: not configured " ++ display pkgid
-    Nothing -> error $ "InstallPlan: no such package " ++ display pkgid
+    _  -> internalError $ "not configured or no such pkg " ++ display pkgid
+
+-- | lookup a package that we expect to be in the configured or failed state
+--
+lookupConfiguredPackage' :: InstallPlan a
+                         -> PackageIdentifier -> Maybe ConfiguredPackage
+lookupConfiguredPackage' plan pkgid =
+  case PackageIndex.lookupPackageId (planIndex plan) pkgid of
+    Just (Configured pkg)  -> Just pkg
+    Just (Failed _ reason) -> Nothing
+    _  -> internalError $ "not configured or no such pkg " ++ display pkgid
 
 -- ------------------------------------------------------------
 -- * Checking valididy of plans