Add stdout/stderr to error messages

[ehmicky]

Fixes #395.

This adds stderr and stdout (in that order) to error messages.

stdout and stderr are not added when they are not available, for example when they are streamed (stdio: 'inherit').

A new property error.shortMessage is added to retrieve error messages without stdout/stderr in case users need shorter error messages.

This PR includes tests, documentation, TypeScript types.

Examples for error.errno were also adjusted to match Node 13 output.

[sindresorhus]

Should we maybe also update the .message docs to explicitly say it includes stdout/stderr? We could also mention which order they're added, that they're separated by a newline and that they're not interleaved.

[ehmicky]

Sounds good. Fixed in 29a4737.