improve error reporting for save resume data for torrents with no metadata

docs/manual.rst

@@ -7439,6 +7439,11 @@ code   symbol                                    description
 ------ ----------------------------------------- -----------------------------------------------------------------
 108    too_frequent_pex                          The peer sent an pex messages too often. This is a possible
                                                  attempt of and attack
+------ ----------------------------------------- -----------------------------------------------------------------
+109    no_metadata                               The operation failed because it requires the torrent to have
+                                                 the metadata (.torrent file) and it doesn't have it yet.
+                                                 This happens for magnet links before they have downloaded the
+                                                 metadata, and also torrents added by URL.
 ====== ========================================= =================================================================
 
 NAT-PMP errors:

include/libtorrent/error_code.hpp

@@ -166,7 +166,7 @@ namespace libtorrent
 			invalid_pex_message,
 			invalid_lt_tracker_message,
 			too_frequent_pex,
-			reserved109,
+			no_metadata,
 			reserved110,
 			reserved111,
 			reserved112,

src/error_code.cpp

@@ -158,7 +158,7 @@ namespace libtorrent
 			"invalid pex message",
 			"invalid lt_tracker message",
 			"pex messages sent too frequent (possible attack)",
-			"",
+			"torrent has no metadata",
 			"",
 			"",
 			"",

src/torrent.cpp

@@ -6007,6 +6007,13 @@ namespace libtorrent
 		TORRENT_ASSERT(m_ses.is_network_thread());
 		INVARIANT_CHECK;
 	
+		if (!valid_metadata())
+		{
+			alerts().post_alert(save_resume_data_failed_alert(get_handle()
+				, errors::no_metadata));
+			return;
+		}
+
 		if (!m_owning_storage.get())
 		{
 			alerts().post_alert(save_resume_data_failed_alert(get_handle()