Separate original error message with a newline

[ehmicky]

When a child process terminated because an error event has been sent to it (or to stdin), which is not so common, we keep the original error message. This PR puts that message on the next line.

At the moment:

> (await execa('wrong command', { reject: false })).message
Command failed with exit code 2 (ENOENT): wrong commandspawn wrong ENOENT

With this PR, this is now:

> (await execa('wrong command', { reject: false })).message
Command failed with exit code 2 (ENOENT): wrong command
spawn wrong ENOENT