Bad error messaged when executable not found

[bazhenov]

Now when tango can not found the baseline executable the error message is kind'a messy:

$ cargo bench -- compare not-existent-path
thread 'main' panicked at ....cargo/registry/src/index.crates.io-6f17d22bba15001f/tango-bench-0.6.0/src/dylib.rs:61:14:
called `Result::unwrap()` on an `Err` value: Unable to open library: not-existent-path

Caused by:
    dlopen(.../sandbox/sum/target/benches/not-existent-path, 0x0005): tried: '.../sandbox/sum/target/benches/not-existent-path' (no such file), '.../sandbox/sum/target/benches/not-existent-path' (no such file), '.../Developer/sandbox/sum/target/benches/not-existent-path' (no such file)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Because it is an easy error to make we need to easy to understand error message that explains what went wrong – which file is not found.