Support for improved error handling >= go 1.13

[simitt]

Add support for

• errors.Unwrap(err error) error
• errors.Cause(err error) error

[segevfiner]

zap opted to avoid handling this by itself. Relying on formatting the error with %+v into errorVerbose , which means the error type needs to implement that to be printed nicely by it. (With no documentation on how to do so properly, and with each library doing it slightly differently, yay Go!). That doesn't mean it can't be done of course. Just something to know about.

Another thing is that due to Go deficiencies it is common to wrap errors with various error wrappers that add things like a stack trace (e.g. github.com/pkg/errors.WrapStack). By just walking the error chain, you will also walk over that type, which delegates to the wrapped error and will cause its message to be printed twice unless it will be handled specially.