sign-file: Fix confusing error messages

When an error occurs, use errx() instead of err() to display the
error message, because openssl has its own error record. When an
error occurs, errno will not be changed, while err() displays the
errno error message. It will cause confusion. For example, when
CMS_add1_signer() fails, the following message will appear:

  sign-file: CMS_add1_signer: Success

errx() ignores errno and does not cause such issue.

Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>