Better handling of TOCLIENT_MEDIA after client is started

This is an attempt to fix the "Assertion '!m_mesh_update_thread.IsRunning()' failed." crash shown in http://pastebin.com/4ctZUC2v. When TOCLIENT_MEDIA is received when all media has already been received, write an error message to errorstream (unless num_files is 0) and return before the mesh update thread assertion.

Since the assertion failure can not reliably be reproduced, I don't know if this patch really fixes it. This needs testing, also in combination with cURL enabled clients and servers.