Refactor shipping rates to assert instead of return error.

Mobile Buy SDK/Mobile Buy SDK/Data/BUYClient.m

@@ -498,10 +498,9 @@ NSString *const BUYClientCustomerAccessToken = @"X-Shopify-Customer-Access-Token
 
 - (NSURLSessionDataTask *)getShippingRatesForCheckout:(BUYCheckout *)checkout completion:(BUYDataShippingRatesBlock)block
 {
-	NSURLSessionDataTask *task = nil;
-	if ([checkout hasToken]) {
+	NSAssert([checkout hasToken], @"Failed to get completion status of checkout. Checkout must have a valid token associated with it.");
 	NSURLComponents *components = [self URLComponentsForCheckoutsAppendingPath:@"shipping_rates" checkoutToken:checkout.token queryItems:@{ @"checkout" : @"" }];
-		task = [self getRequestForURL:components.URL completionHandler:^(NSDictionary *json, NSURLResponse *response, NSError *error) {
+	return [self getRequestForURL:components.URL completionHandler:^(NSDictionary *json, NSURLResponse *response, NSError *error) {
 		NSArray *shippingRates = nil;
 		if (json && !error) {
 			shippingRates = [self.modelManager insertShippingRatesWithJSONArray:json[@"shipping_rates"]];
@@ -510,11 +509,6 @@ NSString *const BUYClientCustomerAccessToken = @"X-Shopify-Customer-Access-Token
 		NSInteger statusCode = [(NSHTTPURLResponse *)response statusCode];
 		block(shippingRates, [BUYClient statusForStatusCode:statusCode error:error], error);
 	}];
-	}
-	else {
-		block(nil, BUYStatusUnknown, [NSError errorWithDomain:kShopifyError code:BUYShopifyError_InvalidCheckoutObject userInfo:nil]);
-	}
-	return task;
 }
 
 #pragma mark - Payments