set d.message on hash check failure

Use Download::hash_error_message() to get a descriptive error string when
hash check fails due to I/O error, instead of relying on errno which could
be zero.

Also fix Manager::receive_hashing_changed() where set_hash_failed(true) was
called without setting d.message when catching local_error during hash
check (e.g. 'too many open files').

src/core/download_list.cc

@@ -492,7 +492,11 @@ DownloadList::hash_done(Download* download) {
 
   if (!download->is_hash_checked()) {
     download->set_hash_failed(true);
-    
+
+    auto& msg = download->download()->hash_error_message();
+    if (!msg.empty())
+      download->set_message(msg);
+
     DL_TRIGGER_EVENT(download, "event.download.hash_failed");
     return;
   }

src/core/manager.cc

@@ -513,6 +513,7 @@ Manager::receive_hashing_changed() {
 
       } else {
         (*itr)->set_hash_failed(true);
+        (*itr)->set_message("Hashing failed: " + std::string(e.what()));
         lt_log_print(torrent::LOG_TORRENT_ERROR, "Hashing failed: %s", e.what());
       }
     }