LocalStore::addToStore(): Ignore exceptions from parseDump()

In the "discard" case (i.e. when the store path already exists
locally), when we call parseDump() from a Finally and it throws an
exception (e.g. if the download of the NAR fails), Nix crashes:

   terminate called after throwing an instance of 'nix::SubstituteGone'
     what():  error: file 'nar/06br3254rx4gz4cvjzxlv028jrx80zg5i4jr62vjmn416dqihgr7.nar.xz' does not exist in binary cache 'http://localhost'
   Aborted (core dumped)

src/libstore/local-store.cc

@@ -1049,7 +1049,11 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
     Finally cleanup = [&]() {
         if (!narRead) {
             NullParseSink sink;
-            parseDump(sink, source);
+            try {
+                parseDump(sink, source);
+            } catch (...) {
+                ignoreException();
+            }
         }
     };