Ensure Github::Import#execute returns false if any errors occured

Signed-off-by: Rémy Coutable <remy@rymai.me>

lib/github/import.rb

@@ -42,7 +42,7 @@ module Github
       puts 'Expiring repository cache...'.color(:aqua) if verbose
       expire_repository_cache
 
-      true
+      errors.empty?
     rescue Github::RepositoryFetchError
       expire_repository_cache
       false