bisect: fix exit code handling to align with git bisect
Change ERROR from 255 to 128 in BisectResult enum. Update comments to clarify the error code behavior. Distinguish build failures (exit 125 - skip commit) from environment failures (exit 128 - abort bisection) in execute_bisection.sh to prevent setup issues from being misclassified as bad commits.
Fixes: 2f7460b0 ("scripts: add execute_bisection.sh for git bisect-based regression detection")
Signed-off-by: Aishwarya TCV aishwarya.tcv@arm.com