Add some debug to show ser_open error

https://github.com/Hamlib/Hamlib/issues/383

src/serial.c

@@ -680,6 +680,10 @@ int ser_open(hamlib_port_t *p)
             ret = OPEN(p->pathname, O_RDWR | O_NOCTTY | O_NDELAY);
         }
     }
+    if (ret == -1)
+    {
+        rig_debug(RIG_DEBUG_ERR, "%s: OPEN failed %d=%s\n", __func__, ret, strerror(errno));
+    }
 
     p->fd = ret;
     return ret;