Add explicit error for missing exec.d paths

[Malax]

I recently messed up packaging my buildpack and all exec.d binaries were not properly packaged. The error messages made it hard to understand what was wrong since the error only complains about "file not found" but doesn't tell you anything about which file.

This PR adds an explicit error for missing exec.d paths so this error-case is much easier to debug. This is technically a breaking change for users that manually match libcnb::Error. Most users won't be affected if they rely on libcnb handling internal errors.

Closes GUS-W-10919490

[edmorley]

Implementing using a conditional rather than a combinator seems a bit simpler, since:

• it avoids the need to wrap in Some()
• means the map_err is no longer required
• means the .clone() is no longer required
• IMO is just generally less nested and fewer words/parens to sift through when reading in this case

Suggested change

	Some(&path)
	.filter(|path| path.exists())
	.ok_or_else(|| WriteLayerError::MissingExecDFile(path.clone()))
	.and_then(|path| {
	fs::copy(path, exec_d_dir.join(name)).map_err(WriteLayerError::IoError)
	})?;
	if !path.exists() {
	return Err(WriteLayerError::MissingExecDFile(path));
	}
	fs::copy(path, exec_d_dir.join(name))?;

However I'm fine with it either way.

[edmorley]

Thank you for fixing this - quick UX win! :-)