Add better handling for empty component assignments

Language/Haskell/GhcMod/Target.hs

@@ -172,7 +172,15 @@ targetGhcOptions crdl sefnmn = do
        let mdlcs = moduleComponents mcs `zipMap` Set.toList sefnmn
            candidates = Set.unions $ map snd mdlcs
 
-       when (Set.null candidates) $
+       let noCandidates = Set.null candidates
+           noModuleHasAnyAssignment = all (Set.null . snd) mdlcs
+
+       if noCandidates && noModuleHasAnyAssignment
+          then do
+            gmLog GmWarning "" $ strDoc $ "Could not find a componenet assignment, falling back to sandbox only project options."
+            sandboxOpts crdl
+          else do
+            when noCandidates $
               throwError $ GMECabalCompAssignment mdlcs
 
             let cn = pickComponent candidates