apps: print an error for a non-positive genrsa bits argument

Previously a negative or zero numbits argument made genrsa exit
with a failure status without printing any error message.

Assisted-by: Claude:claude-fable-5
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.foundation>
Merge-date: Mon Sep  7 14:15:50 2026
Merged-from: https://github.com/openssl/openssl/pull/32592