Switched internal error result property to use generic Failure type

Source/AFResult.swift

@@ -36,7 +36,7 @@ extension AFResult {
     }
 
     /// Returns the associated error value if the result is a failure, `nil` otherwise.
-    var error: Error? {
+    var error: Failure? {
         guard case .failure(let error) = self else { return nil }
         return error
     }

Tests/AFResult+Alamofire.swift

@@ -40,7 +40,7 @@ extension AFResult {
         return value
     }
 
-    var error: Error? {
+    var error: Failure? {
         guard case .failure(let error) = self else { return nil }
         return error
     }