[apt-pkg/sourcelist.cc] only display the warning about missing sources.list

file, if no SourceListEntry exists (e.g. sources.list.d dir does not exists
or it exists but does not include a .list-file with valid entries)

apt-pkg/sourcelist.cc

@@ -157,10 +157,15 @@ bool pkgSourceList::ReadMainList()
    // CNC:2003-11-28 - Entries in sources.list have priority over
    //                  entries in sources.list.d.
    string Main = _config->FindFile("Dir::Etc::sourcelist");
+   string noSourceMsg;
    if (FileExists(Main) == true)
       Res &= ReadAppend(Main);
    else
+   {
+      // only print the warning if we can't load a valid sourcefile in the end
       _error->WarningE("FileExists",_("Unable to read %s"),Main.c_str());
+      _error->PopMessage(noSourceMsg);
+   }
 
    string Parts = _config->FindDir("Dir::Etc::sourceparts");
    if (FileExists(Parts) == true)
@@ -168,6 +173,9 @@ bool pkgSourceList::ReadMainList()
    else
       _error->WarningE("FileExists",_("Unable to read %s"),Parts.c_str());
 
+   if (SrcList.empty() == true && noSourceMsg.empty() == false)
+      _error->Warning(noSourceMsg.c_str());
+
    return Res;
 }
 									/*}}}*/