Add extra ERROR on mailbox receive timeout to improv diagnostics possibilities

soem/ethercatmain.c

@@ -1097,7 +1097,8 @@ int ecx_mbxreceive(ecx_contextt *context, uint16 slave, ec_mbxbuft *mbx, int tim
       }
       else /* no read mailbox available */
       {
-          wkc = 0;
+         if (wkc > 0)
+            wkc = EC_TIMEOUT;
       }
    }
 

soem/ethercattype.h

@@ -45,6 +45,8 @@ extern "C"
 #define EC_ERROR              -3
 /** return value too many slaves */
 #define EC_SLAVECOUNTEXCEEDED -4
+/** return value request timeout */
+#define EC_TIMEOUT            -5
 /** maximum EtherCAT frame length in bytes */
 #define EC_MAXECATFRAME    1518
 /** maximum EtherCAT LRW frame length in bytes */