run_benchmark: exit non-zero on import failure

[fallintoplace]

Summary

• Make user benchmark import failures in run_benchmark.py exit with a non-zero status (SystemExit(1)) so automation doesn’t treat them as success.
• Keep benchmark execution failures bubbling up naturally so real runtime errors remain visible.
• Add regression coverage in test_run_benchmark.py for both behaviors.

Why this matters

When benchmark imports fail, the script currently only logs a traceback and returns success, which can let broken benchmark runs pass CI checks.