Don't error if package index is missing

All of the packages Cabal needs may already be present in the local
package database; we don't know yet!

This is a partial revert of haskell#8944.

See: haskell#8944 (comment)

cabal-install/src/Distribution/Client/IndexUtils.hs

@@ -460,8 +460,8 @@ readRepoIndex verbosity repoCtxt repo idxState =
       if isDoesNotExistError e
         then do
           case repo of
-            RepoRemote{..} -> dieWithException verbosity $ MissingPackageList repoRemote
-            RepoSecure{..} -> dieWithException verbosity $ MissingPackageList repoRemote
+            RepoRemote{..} -> warn verbosity $ exceptionMessageCabalInstall $ MissingPackageList repoRemote
+            RepoSecure{..} -> warn verbosity $ exceptionMessageCabalInstall $ MissingPackageList repoRemote
             RepoLocalNoIndex local _ ->
               warn verbosity $
                 "Error during construction of local+noindex "