Downgrade 'invalid result from curve25519 handshake: 4' warning

Also, refactor the way we handle failed handshakes so that this
warning doesn't propagate itself to "onion_skin_client_handshake
failed" and "circuit_finish_handshake failed" and
"connection_edge_process_relay_cell (at origin) failed."

Resolves warning from 9635.

src/or/command.c

@@ -398,7 +398,6 @@ command_process_created_cell(cell_t *cell, channel_t *chan)
     log_debug(LD_OR,"at OP. Finishing handshake.");
     if ((err_reason = circuit_finish_handshake(origin_circ,
                                         &extended_cell.created_cell)) < 0) {
-      log_warn(LD_OR,"circuit_finish_handshake failed.");
       circuit_mark_for_close(circ, -err_reason);
       return;
     }