Add generic :custom_error on HTTP/2 frames (#445)

Author: kaiserdan

lib/mint/http2/frame.ex

@@ -472,4 +472,7 @@ defmodule Mint.HTTP2.Frame do
     defp humanize_error_code(unquote(code)), do: unquote(human_code)
     defp dehumanize_error_code(unquote(human_code)), do: unquote(code)
   end
+
+  defp humanize_error_code(code), do: {:custom_error, code}
+  defp dehumanize_error_code({:custom_error, code}), do: code
 end

test/mint/http2/frame_test.exs

@@ -294,7 +294,10 @@ defmodule Mint.HTTP2.FrameTest do
       :connect_error,
       :enhance_your_calm,
       :inadequate_security,
-      :http_1_1_required
+      :http_1_1_required,
+      {:custom_error, 0x11},
+      {:custom_error, 0xFF},
+      {:custom_error, 70007}
     ])
   end
 end