Enhance error output on missing Cargo.lock

[chrysn]

This changes the previously generic error ("File not found") to produce this when a library crate uses built's set_dependencies gets built:

error: failed to run custom build command for `leafcrate v0.1.0 (/tmp/leafcrate)`

Caused by:
  process didn't exit successfully: `/tmp/leafcrate/target/debug/build/leafcrate-1c09147a939b59c4/build-script-build` (exit code: 101)
  --- stderr
  thread 'main' panicked at 'Failed to acquire build-time information: Custom { kind: NotFound, error: "No Cargo.lock file found, see set_dependencies documentation" }', build.rs:9:10
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Contributes-To: #32

[lukaslueg]

Thanks for the PR!

I think there is a problem here because the current code maps any kind of error and replaces the message with one that specifically refers to "no found". We could still improve the situation by printing a longer message to stderr, which Cargo will replay. Something to the tune of

fs::File::open(manifest_location.join("Cargo.lock"))
    .map_err(|e| {
            eprintln!("Failed to open Cargo.lock. This information is only available for top-level packages. See the documentation for `built::Options::set_dependencies`.");
            e
        })?
        .read_to_string(&mut lock_buf)?;