yarn publish errors are opaque

[OliverJAsh]

Description

yarn publish errors are opaque.

Steps to reproduce

1. Define a prepublishOnly script that always fails
2. Run np (in Yarn mode)

Expected behavior

It should be clear why yarn publish failed, i.e. what error did prepublishOnly result in?

Actual behaviour

✖ Error publishing package:
Command failed with exit code 2: yarn publish

There's no mention of why exactly yarn publish failed.

Environment

np - 5.2.1
Node.js - 12.8.1
npm - 6.10.2
Git - 2.23.0
OS - macOS 10.15.2

[sindresorhus]

This should be fixed by 818ff4b#diff-b9cfc7f2cdf78a7f4b91a753d10865a2R40. Older execa versions did not include stdout/stderr in error messages.