Try to give a more helpful error message when we can't read LBI.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Cabal/tests/PackageTests.hs

@@ -210,6 +210,10 @@ getPersistBuildConfig_ filename = do
     eLBI <- try $ getConfigStateFile filename
     case eLBI of
       Left (ConfigStateFileBadVersion _ _ (Right lbi)) -> return lbi
-      Left (ConfigStateFileBadVersion _ _ (Left err)) -> throw err
+      Left (ConfigStateFileBadVersion _ _ (Left err))
+        -> error $ "We couldn't understand the build configuration.  Try " ++
+                   "building Cabal with a more recent version of itself " ++
+                   "and then running the test suite.\n\nOriginal error: " ++
+                   show err
       Left err -> throw err
       Right lbi -> return lbi