Allow custom error messages on claim validation

lib/joken.ex

@@ -34,7 +34,7 @@ defmodule Joken do
     - Signer configuration
     - Hooks
 
-  The portable token claims configuration is a map of binary keys to `Joken.Claim` structs and is used 
+  The portable token claims configuration is a map of binary keys to `Joken.Claim` structs and is used
   to dynamically generate and validate tokens.
 
   A signer is an instance of `Joken.Signer` that encapsulates the algorithm and the key configuration
@@ -400,7 +400,8 @@ defmodule Joken do
             """
           end)
 
-          {:halt, {:error, message: "Invalid token", claim: key, claim_val: claim_val}}
+          message = Keyword.get(config[key].options, :message, "Invalid token")
+          {:halt, {:error, message: message, claim: key, claim_val: claim_val}}
       end
     end)
     |> case do

test/joken_test.exs

@@ -1,7 +1,7 @@
 defmodule JokenTest do
   use ExUnit.Case, async: true
   import ExUnit.CaptureLog
-  import Joken.Config, only: [add_claim: 4, default_claims: 1]
+  import Joken.Config, only: [add_claim: 4, add_claim: 5, default_claims: 1]
   alias Joken.CurrentTime.Mock
 
   setup do
@@ -78,6 +78,24 @@ defmodule JokenTest do
                "Claim %{\"iss\" => \"someone\"} did not pass validation.\n\nCurrent time: "
     end
 
+    test "claim attaches custom error message" do
+      custom_error_message = "Someone should not be there"
+
+      token_config =
+        %{}
+        |> add_claim(
+          "iss",
+          fn -> "not someone" end,
+          fn val ->
+            val == "not someone"
+          end,
+          message: custom_error_message
+        )
+
+      assert {:error, [message: custom_error_message, claim: "iss", claim_val: "someone"]} ==
+               Joken.validate(token_config, %{"iss" => "someone"}, %{})
+    end
+
     test "can make multi claim validation" do
       token_config = %{} |> add_claim("claim1", nil, &(&1 == &2["claim2"]))