Refactor get products page to assert instead of return error.

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

@@ -221,8 +221,8 @@ NSString *const BUYClientCustomerAccessToken = @"X-Shopify-Customer-Access-Token
 
 - (NSURLSessionDataTask *)getProductsPage:(NSUInteger)page inCollection:(NSNumber *)collectionId sortOrder:(BUYCollectionSort)sortOrder completion:(BUYDataProductListBlock)block
 {
-	NSURLSessionDataTask *task = nil;
-	if (collectionId) {
+	NSAssert(collectionId, @"Failed to get products page. Invalid collectionID.");
+	
 	NSURLComponents *components = [self URLComponentsForChannelsAppendingPath:kBUYClientPathProductPublications
 																   queryItems:@{
 																				@"collection_id" : collectionId.stringValue,
@@ -231,7 +231,7 @@ NSString *const BUYClientCustomerAccessToken = @"X-Shopify-Customer-Access-Token
 																				@"sort_by" : [BUYCollection sortOrderParameterForCollectionSort:sortOrder]
 																				}];
 	
-		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 *products = nil;
 		if (json && !error) {
@@ -239,12 +239,6 @@ NSString *const BUYClientCustomerAccessToken = @"X-Shopify-Customer-Access-Token
 		}
 		block(products, page, [self hasReachedEndOfPage:products] || error, error);
 	}];
-	}
-	else {
-		block(nil, 0, NO, [NSError errorWithDomain:kShopifyError code:BUYShopifyError_NoCollectionIdSpecified userInfo:nil]);
-	}
-	
-	return task;
 }
 
 #pragma mark - Helpers