Go-multierror is a package for Go that provides a mechanism for representing a
list of error values as a single error.

This allows a function in Go to return an error that might actually be a list of
errors. If the caller knows this, they can unwrap the list and access the
errors. If the caller doesn't know, the error formats to a nice human-readable
format.

Go-multierror implements the errwrap interface so that it can be used with that
library, as well.