Make the qa check return an exit code reporting the status

cabal-install/Hackage/Check.hs

@@ -23,7 +23,7 @@ import Distribution.PackageDescription.Configuration ( flattenPackageDescription
 import Distribution.Verbosity ( Verbosity )
 import Distribution.Simple.Utils ( defaultPackageDesc )
 
-check :: Verbosity -> IO ()
+check :: Verbosity -> IO Bool
 check verbosity = do
     pdfile <- defaultPackageDesc verbosity
     ppd <- readPackageDescription verbosity pdfile
@@ -78,3 +78,4 @@ check verbosity = do
     when (null packageChecks) $ do
         putStrLn "No errors or warnings could be found in the package."
 
+    return (null packageChecks)

cabal-install/Main.hs

@@ -202,4 +202,7 @@ checkAction :: Flag Verbosity -> [String] -> IO ()
 checkAction verbosityFlag extraArgs = do
   unless (null extraArgs) $ do
     die $ "'check' doesn't take any extra arguments: " ++ unwords extraArgs
-  Check.check (fromFlag verbosityFlag)
+  allOk <- Check.check (fromFlag verbosityFlag)
+  if allOk
+    then exitWith ExitSuccess
+    else exitWith (ExitFailure 1)