Extends Storer interface for all methods to also return an error

In order to implement the Storer interface, all methods must now return
and error.

The value property in Puppy has had its type changed to an int to
represent a numerical cent figure so that it may be properly throw an
error if the value supplied is less than 0.

Lab 07 (anz-bank#589)

07_errors/patrickmarabeas/types.go

@@ -1,21 +1,19 @@
 package main
 
-import (
-	"sync"
-)
+import "sync"
 
 type Puppy struct {
 	ID    int
 	Breed string
 	Color string
-	Value string
+	Value int // cents
 }
 
 type Storer interface {
-	Create(puppy Puppy) int
-	Read(ID int) Puppy
-	Update(ID int, puppy Puppy) bool
-	Destroy(ID int) bool
+	Create(puppy Puppy) (int, error)
+	Read(ID int) (Puppy, error)
+	Update(ID int, puppy Puppy) (bool, error)
+	Destroy(ID int) (bool, error)
 }
 
 type MapStore struct {