buffer: Fail early and print diagnostic message

When /tmp/mist/ was created before as a different user, and MistBuffer was
started, it would print "End of input file - buffer shutting down". This is very
confusing, therefore fail earlier and print a more useful message even if DEBUG
is disabled.

src/buffer.cpp

@@ -188,6 +188,10 @@ namespace Buffer{
     std::string name = argv[1];
 
     SS = Socket::makeStream(name);
+    if (!SS.connected()) {
+      perror("Could not create stream socket");
+      return 1;
+    }
     thisStream = Stream::get();
     thisStream->setName(name);
     Socket::Connection incoming;