faircode benchmark leaks a raw traceback for a missing manifest path #616

Where: faircode/cli.py's benchmark handler only catches ImportError/ValueError around run_benchmark(); run_benchmark() itself (faircode/benchmark.py) wraps each manifest's processing in except (yaml.YAMLError, KeyError, ValueError) specifically "to avoid a raw, manifest-path-less traceback all the way to the CLI" per its own comment - but load_manifest() (faircode/manifest.py) does a bare open(path), raising FileNotFoundError, which isn't in that tuple.

Repro:

$ python3 -m faircode.cli benchmark nonexistent/audit.yaml
Traceback (most recent call last):
  ...
  File ".../faircode/manifest.py", line 211, in load_manifest
    with open(path) as f:
FileNotFoundError: [Errno 2] No such file or directory: 'nonexistent/audit.yaml'

Every other file-reading path in this CLI (_read_or_exit) catches FileNotFoundError cleanly with a error: file not found: ... message - this is the one path that doesn't, breaking the exact promise run_benchmark()'s own comment makes.

Fix direction: add FileNotFoundError to run_benchmark()'s per-manifest except tuple (or to the CLI handler's), converting it to the same clean error: ... style as everywhere else.