Implemented some test cases for the Fetcher class and the URL+Device extension.

Sources/Persistence/Protocols/Service.swift

@@ -27,10 +27,10 @@ public protocol Service {
     
     /// Save a given context.
     /// - Parameter context: A `NSManagedObjectContext` context instance to save.
-    func save(context: NSManagedObjectContext)
+    func save(context: NSManagedObjectContext) throws
     
     /// Save a given child context as well as its respective parent context.
     /// - Parameter context: A child `NSManagedObjectContext` context instance to save.
-    func save(childContext context: NSManagedObjectContext)
+    func save(childContext context: NSManagedObjectContext) throws 
     
 }