cargo-inspector: don't print the traceback if a file is missing
    
    $ cargo-inspector -n zola
    Traceback (most recent call last):
      File "/usr/bin/cargo-inspector", line 33, in <module>
        sys.exit(load_entry_point('rust2rpm==18', 'console_scripts', 'cargo-inspector')())
      File "/home/zbyszek/python/rust2rpm/rust2rpm/inspector.py", line 78, in main
        files = [_cargo_toml(args.path, f) for f in files]
      File "/home/zbyszek/python/rust2rpm/rust2rpm/inspector.py", line 78, in <listcomp>
        files = [_cargo_toml(args.path, f) for f in files]
      File "/home/zbyszek/python/rust2rpm/rust2rpm/inspector.py", line 50, in _cargo_toml
        raise FileNotFoundError(f'Cargo.toml not found for binary {binary_or_cargo_toml}')
    FileNotFoundError: Cargo.toml not found for binary zola
    
    becomes
    
    $ cargo-inspector -n zola
    FileNotFoundError: Cargo.toml not found for binary zola