Handle errors from streams passed to the std* option

[ehmicky]

Users can pass Node.js streams to the stdin/stdout/stderr option.

If that stream fail, the promise returned by Execa should fail. This currently happens for any stream created under-the-hood when passing special values to the std* option. For example, when passing {stdout: {file: 'file.txt'}} and a file error happens, Execa will reject with it.

However, this error handling currently only happens when the Node.js stream is passed as an array like {stdout: [stream, 'pipe']} but not when it is passed on its own like {stdout: stream}. We should fix this.