Downgrade a PROTOCOL_WARN log message.

This one happens every time we get a failure from
circuit_receive_relay_cell -- but for all the relevant failing cases
in that function, we already log in that function.

This resolves one case of #40400.  Two cases remain.

changes/bug40400

@@ -0,0 +1,6 @@
+  o Minor bugfixes (usability):
+    - Do not log "circuit_receive_relay_cell failed" at LOG_PROTOCOL_WARN;
+      instead log it at DEBUG. In every case where we would want to log
+      this as a protocol warning, we are already logging another warning
+      from inside circuit_receive_relay_cell. Fixes part of bug 40400;
+      bugfix on 0.1.1.9.

src/core/or/command.c

@@ -563,7 +563,7 @@ command_process_relay_cell(cell_t *cell, channel_t *chan)
   }
 
   if ((reason = circuit_receive_relay_cell(cell, circ, direction)) < 0) {
-    log_fn(LOG_PROTOCOL_WARN,LD_PROTOCOL,"circuit_receive_relay_cell "
+    log_fn(LOG_DEBUG,LD_PROTOCOL,"circuit_receive_relay_cell "
            "(%s) failed. Closing.",
            direction==CELL_DIRECTION_OUT?"forward":"backward");
     /* Always emit a bandwidth event for closed circs */