dataman: improve error reporting

src/modules/dataman/dataman.cpp

@@ -637,7 +637,7 @@ _file_read(dm_item_t item, unsigned index, void *buf, size_t count)
 
 	/* Check for read error */
 	if (len < 0) {
-		return -1;
+		return -errno;
 	}
 
 	/* A zero length entry is a empty entry */

src/modules/mavlink/mavlink_mission.cpp

@@ -120,7 +120,7 @@ MavlinkMissionManager::init_offboard_mission()
 			_current_seq = mission_state.current_seq;
 
 		} else if (ret < 0) {
-			PX4_ERR("offboard mission init failed (%i)", errno);
+			PX4_ERR("offboard mission init failed (%i)", ret);
 		}
 
 		load_geofence_stats();