test/e2e/s3: attach stdout & stderr to exec() errors (#1340)

It may not be immediately obvious that stdout and stderr are actually available when a call to `child_process.exec()` fails.  Attaching them here should bring attention to this, and aid future debugging.

test/e2e/s3/test.js

@@ -403,8 +403,12 @@ function cli(cmd) {
   const env = { ..._.pick(process.env, 'PATH'), NODE_CONFIG_ENV:'s3-dev' };
 
   const promise = new Promise((resolve, reject) => {
-    const child = exec(cmd, { env, cwd:'../../..' }, (err, stdout) => {
-      if (err) return reject(err);
+    const child = exec(cmd, { env, cwd:'../../..' }, (err, stdout, stderr) => {
+      if (err) {
+        err.stdout = stdout; // eslint-disable-line no-param-reassign
+        err.stderr = stderr; // eslint-disable-line no-param-reassign
+        return reject(err);
+      }
 
       const res = stdout.toString().trim();
       log.debug('cli()', 'returned:', res);