Improve error message if package file is not found

[bringert]

In 0.3.0, if you give a non-existing file path to buildfs run, the error message doesn't explain what file isn't found. In this example thebuild_script_x.toml file doesn't exist.

$ buildfs run -o debian.ext4 build_script_x.toml
thread 'main' panicked at /usr/local/cargo/registry/src/index.crates.io-6f17d22bba15001f/buildfs-0.3.0/src/package.rs:14:14:
Could not inspect file/directory metadata: Os { code: 2, kind: NotFound, message: "No such file or directory" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

It would have been nicer if the error message included the name of the file that wasn't found.