tools/xen-ucode: return correct exit code on failed microcode update

tools/xen-ucode: return correct exit code on failed microcode update

Otherwise it's difficult to know if operation failed inside the automation.

While at it, also switch to returning 1 and 2 instead of errno to avoid
incompatibilies between errno and special exit code numbers.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Release-acked-by: Paul Durrant <paul@xen.org>
Reviewed-by: Igor Druzhinin <igor.druzhinin@citrix.com>