qapi-commands: Fix gen_err_check(e) for e and e != 'local_err'

gen_err_check() hard-codes 'local_err' instead of substituting the
argument.  Currently harmless, since all callers pass either None or
'local_err'.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>