Add errdefs package

Providers should use this package so the virtual kubelet core
controllers can understand the errors produced from the provider code.

errdefs/notfound.go

@@ -0,0 +1,65 @@
+package errdefs
+
+import (
+	"errors"
+	"fmt"
+)
+
+// NotFound is an error interface which denotes whether the opration failed due
+// to a the resource not being found.
+type ErrNotFound interface {
+	NotFound() bool
+	error
+}
+
+type notFoundError struct {
+	error
+}
+
+func (e *notFoundError) NotFound() bool {
+	return true
+}
+
+func (e *notFoundError) Cause() error {
+	return e.error
+}
+
+// AsNotFound wraps the passed in error to make it of type ErrNotFound
+//
+// Callers should make sure the passed in error has exactly the error message
+// it wants as this function does not decorate the message.
+func AsNotFound(err error) error {
+	if err == nil {
+		return nil
+	}
+	return &notFoundError{err}
+}
+
+// NotFound makes an ErrNotFound from the provided error message
+func NotFound(msg string) error {
+	return &notFoundError{errors.New(msg)}
+}
+
+// NotFoundf makes an ErrNotFound from the provided error format and args
+func NotFoundf(format string, args ...interface{}) error {
+	return &notFoundError{fmt.Errorf(format, args...)}
+}
+
+// IsNotFound determines if the passed in error is of type ErrNotFound
+//
+// This will traverse the causal chain (`Cause() error`), until it finds an error
+// which implements the `NotFound` interface.
+func IsNotFound(err error) bool {
+	if err == nil {
+		return false
+	}
+	if e, ok := err.(ErrNotFound); ok {
+		return e.NotFound()
+	}
+
+	if e, ok := err.(causal); ok {
+		return IsNotFound(e)
+	}
+
+	return false
+}