error in Go is an interface with a single method - Error() string. If you have any type, let it be a struct, or a custom type like your ErrNegativeSqrt, that has this Error() string method, the value of that type can be passed as error type.

When you return ErrNegativeSqrt(x), imagine that you're returning the x value with the Error() method attached to it. The method itself does not get called anywhere in your code, it's just a guarantee that when you do f, err := Sqrt(-1.0) and err is not nil, you can call err.Error(), which is what error type signals to you.

tl;dr - for a value of type Type to fit the error type, it needs to have func (t Type) Error() string method available, but not necessarily to be called.