Panics if the current directory is not a valid cargo package

[jyn514]

$ cargo metadata --format-version 1
error: could not find `Cargo.toml` in `/home/joshua/src` or any parent directory
$ cargo deadlinks
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error("EOF while parsing a value", line: 1, column: 0)', src/libcore/result.rs:1188:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

Ideally it would pass through the error from cargo, but failing that it should at least say something like 'not a valid cargo package'.

Note that there's no inherent reason this needs to be fatal, since if you pass --dir it works fine. So maybe the error should say 'error: either pass --dir or run this in a valid cargo package'.