dashdemux: return the error to the source immediately to stop faster

When flushing, this will prevent dashdemux from trying to download more
fragments or more chunks of the same fragment before stopping.

Also improves the error handling to not transform everything non-ok into
an error.

https://bugzilla.gnome.org/show_bug.cgi?id=734014

ext/dash/gstdashdemux.c

@@ -1974,11 +1974,10 @@ _src_chain (GstPad * pad, GstObject * parent, GstBuffer * buffer)
 
     /* TODO properly stop tasks */
     /* gst_hls_demux_pause_tasks (demux); */
+    g_cond_signal (&stream->fragment_download_cond);
   }
 
-  /* avoid having the source handle the same error again */
   stream->last_ret = ret;
-  ret = GST_FLOW_OK;
 
   return ret;
 }
@@ -2317,7 +2316,6 @@ static GstFlowReturn
 gst_dash_demux_stream_get_next_fragment (GstDashDemuxStream * stream,
     GstClockTime * ts)
 {
-  GstFlowReturn ret = GST_FLOW_OK;
   GstDashDemux *demux = stream->demux;
 
   if (stream->stream_eos)
@@ -2358,11 +2356,11 @@ gst_dash_demux_stream_get_next_fragment (GstDashDemuxStream * stream,
 
   demux->end_of_period = FALSE;
 
-  if (stream->last_ret < GST_FLOW_EOS) {
-    GST_WARNING_OBJECT (stream->pad, "Failed to download fragment");
-    return GST_FLOW_ERROR;
+  if (stream->last_ret != GST_FLOW_OK) {
+    GST_WARNING_OBJECT (stream->pad, "Failed to download fragment: %s",
+        gst_flow_get_name (stream->last_ret));
   }
-  return ret;
+  return stream->last_ret;
 }
 
 static void