Reproduction:
1. Create new configuration
2. Add step which fail
3. Add step "Execute Script" with Script:
${groovy:
logger.info("FAILED: " + build.isFailed().toString())
logger.info("HAS ERRORS: " + build.hasErrors().toString())}
4. Run build
5. Look to logs and You will see:
12:13:55,926 [master>LOG STATUS@machine:8810] INFO - FAILED: false
12:13:55,926 [master>LOG STATUS@machine:8810] INFO - HAS ERRORS: false

build.isFailed() is false although first step failed.