Return no data error as atom instead of string

lib/feed_parser.ex

@@ -22,11 +22,11 @@ defmodule FeedParser do
   An extended set of parsers may also be provided, otherwise the default set (supporting RSS 2.0, Atom, JSON Feed, and RSS-in-JSON) will be used. Parsers are modules that implement the `FeedParser.Parser` behaviour.
   """
   @spec parse(data :: String.t(), content_type :: String.t(), parsers :: [module()]) ::
-          {:ok, feed :: FeedParser.Feed.t()} | {:error, reason :: String.t()}
+          {:ok, feed :: FeedParser.Feed.t()} | {:error, reason :: term()}
   def parse(data, content_type, parsers \\ @default_parsers)
 
   def parse(nil, _, _) do
-    {:error, "no data"}
+    {:error, :no_data}
   end
 
   def parse(data, content_type, parsers) when is_binary(data) do