Include stdout/stderr in error.message

[sindresorhus]

We removed it in execa v2 for some reason, but it's very useful.

The lack of it leads to a lot of annoyance and makes it harder for users to see what actually went wrong...

It also makes it harder to do test assertions like this:

test('missing file operands', async t => {
	await t.throwsAsync(execa('./cli.js'), /`source` and `destination` required/);
});

Open questions:

• Should we include stdout or only stderr?
• Should we truncate it at some length?

[ehmicky]

Following from some discussion with @sindresorhus, we thought of the following solution:

• error.message should include prefixes (e.g. Command failed with exit code) and be appended with stdout and stderr (if they exist)
• error.shortMessage should include prefixes but not stdout/stderr
• error.originalMessage should include neither prefixes nor stdout/stderr

This would be a breaking change.