lua: fix error handling

lua: fix error handling

scan-build from clang version 9 has reported following issues:

 uci.c:389:3: warning: Value stored to 'err' is never read
                err = UCI_ERR_INVAL;
                ^     ~~~~~~~~~~~~~
 uci.c:393:3: warning: Value stored to 'err' is never read
                err = UCI_ERR_NOTFOUND;
                ^     ~~~~~~~~~~~~~~~~
 uci.c:417:4: warning: Value stored to 'err' is never read
                        err = UCI_ERR_INVAL;
                        ^     ~~~~~~~~~~~~~
 uci.c:524:3: warning: Value stored to 'err' is never read
                err = UCI_ERR_INVAL;
                ^     ~~~~~~~~~~~~~
 uci.c:533:3: warning: Value stored to 'err' is never read
                err = UCI_ERR_INVAL;
                ^     ~~~~~~~~~~~~~
 uci.c:565:4: warning: Value stored to 'err' is never read
                        err = UCI_ERR_INVAL;
                        ^     ~~~~~~~~~~~~~
 uci.c:575:3: warning: Value stored to 'err' is never read
                err = UCI_ERR_INVAL;
                ^     ~~~~~~~~~~~~~
 uci.c:584:3: warning: Value stored to 'err' is never read
                err = UCI_ERR_INVAL;
                ^     ~~~~~~~~~~~~~
 uci.c:642:3: warning: Value stored to 'err' is never read
                err = UCI_ERR_INVAL;
                ^     ~~~~~~~~~~~~~
 uci.c:651:3: warning: Value stored to 'err' is never read
                err = UCI_ERR_INVAL;
                ^     ~~~~~~~~~~~~~

Signed-off-by: Petr Štetiar <ynezz@true.cz>