Better '$' error handling

src/cli.ts

@@ -3,7 +3,7 @@ import {exec, execSync} from 'child_process';
 export function $(str: TemplateStringsArray, ...args: string[]): Promise<string> {
 	let cmd = str.reduce((acc, part, i) => acc + part + (args[i] || ''), '');
 	return new Promise((res, rej) => exec(cmd, (err, stdout, stderr) => {
-		if(err || stderr) return rej(err || stderr);
+		if(err) return rej(stderr || err);
 		return res(stdout);
 	}))
 }