Error interface type

What is an error in Go? An error in Go is a value. Here’s a quote from Rob Pike, one of the pivotal pioneers of Go:

“Values can be programmed, and since errors are values, errors can be programmed. Errors are not like exceptions. There’s nothing special about them, whereas an unhandled exception can crash your program.”

Since errors are values, they can be passed into a function, returned from a function, and evaluated just like any other value in Go.

An error in Go is anything that implements the error interface. Interfaces will be explained in detail in the following chapter, so we will keep the details light in this chapter concerning interface references. We need to look at some fundamental aspects that make up the error type in Go. To be an error type in Go, it must first satisfy type error interface:

//https://golang.org/pkg/builtin/#error
type error interface {
  Error() string
}

The wonderful thing about...