Fix bitwalker#398 - handle FormatError with non-string reasons

lib/format/format_error.ex

@@ -2,11 +2,15 @@ defmodule Timex.Format.FormatError do
   @moduledoc """
   Used for errors encountered during string formatting.
   """
-  alias Timex.Format.FormatError
-
   defexception message: "Invalid format!"
 
+  def exception([message: message]) when is_binary(message) do
+    %__MODULE__{message: message}
+  end
   def exception([message: message]) do
-    %FormatError{message: message}
+    %__MODULE__{message: "Invalid format: #{inspect message}"}
+  end
+  def exception(err) do
+    %__MODULE__{message: "Invalid format: #{inspect err}"}
   end
 end