discover/boot: abort kexec on any error from validation

gpg_validate_boot_files() can return error codes for a variety of
reasons but kexec_load only aborts for signature or decryption failure.
In any other failure case like unable to open LOCKDOWN_FILE or do the
secure copy the validation is bypassed by an early return but kexec_load
does not abort.

Signed-off-by: Brett Grandbois <brett.grandbois@opengear.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>