Include actual exception message with custom exceptions

Author: bf4

CHANGELOG.md

@@ -6,6 +6,8 @@ Breaking changes:
 - [#1662](https://github.com/rails-api/active_model_serializers/pull/1662) Drop support for Rails 4.0 and Ruby 2.0.0. (@remear)
 
 Features:
+- [#1697](https://github.com/rails-api/active_model_serializers/pull/1697) Include actual exception message with custom exceptions;
+  `Test::Schema` exceptions are now `Minitest::Assertion`s. (@bf4)
 - [#1699](https://github.com/rails-api/active_model_serializers/pull/1699) String/Lambda support for conditional attributes/associations (@mtsmfm)
 - [#1687](https://github.com/rails-api/active_model_serializers/pull/1687) Only calculate `_cache_digest` (in `cache_key`) when `skip_digest` is false. (@bf4)
 - [#1647](https://github.com/rails-api/active_model_serializers/pull/1647) Restrict usage of `serializable_hash` options

lib/active_model_serializers/test/schema.rb

@@ -32,7 +32,7 @@ def initialize(schema_path, response, message)
         def call
           json_schema.expand_references!(store: document_store)
           status, errors = json_schema.validate(response_body)
-          @message ||= errors.map(&:to_s).to_sentence
+          @message = [message, errors.map(&:to_s).to_sentence].compact.join(': ')
           status
         end
 

test/active_model_serializers/test/schema_test.rb

@@ -54,13 +54,15 @@ def test_that_raises_a_minitest_error_with_a_invalid_schema
 
       def test_that_raises_error_with_a_custom_message_with_a_invalid_schema
         message = 'oh boy the show is broken'
+        exception_message = "#/name: failed schema #/properties/name: For 'properties/name', \"Name 1\" is not an integer. and #/description: failed schema #/properties/description: For 'properties/description', \"Description 1\" is not a boolean."
+        expected_message = "#{message}: #{exception_message}"
 
         get :show
 
         error = assert_raises Minitest::Assertion do
           assert_response_schema(nil, message)
         end
-        assert_equal(message, error.message)
+        assert_equal(expected_message, error.message)
       end
 
       def test_that_assert_with_a_custom_schema
@@ -102,14 +104,14 @@ def test_that_assert_with_a_custom_schema_directory
       end
 
       def test_with_a_non_existent_file
-        expected_message = 'No Schema file at test/support/schemas/non-existent.json'
+        message = 'No Schema file at test/support/schemas/non-existent.json'
 
         get :show
 
         error = assert_raises ActiveModelSerializers::Test::Schema::MissingSchema do
           assert_response_schema('non-existent.json')
         end
-        assert_equal(expected_message, error.message)
+        assert_equal(message, error.message)
       end
 
       def test_that_raises_with_a_invalid_json_body