Allow custom error messages

* Adds ability to override key in resource to surface different error messages

Author: fzf

lib/json_api_client/resource.rb

@@ -568,10 +568,14 @@ def relationships_for_serialization
       relationships.as_json_api
     end
 
+    def error_message_for(error)
+      error.error_msg
+    end
+
     def fill_errors
       last_result_set.errors.each do |error|
         key = self.class.key_formatter.unformat(error.error_key)
-        errors.add(key, error.error_msg)
+        errors.add(key, error_message_for(error))
       end
     end
   end

test/unit/error_collector_test.rb

@@ -215,6 +215,48 @@ def test_can_handle_parameter_error
     assert_equal "include", error.source_parameter
   end
 
+  describe 'custom error_msg_key' do
+    class CustomErrorArticle < TestResource
+      def error_message_for(error)
+        error.detail
+      end
+    end
+
+    def test_can_handle_custom_parameter_error
+      stub_request(:post, "http://example.com/custom_error_articles")
+          .with(headers: {content_type: "application/vnd.api+json", accept: "application/vnd.api+json"}, body: {
+              data: {
+                  type: "custom_error_articles",
+                  attributes: {
+                      title: "Rails is Omakase",
+                      email_address: "bar"
+                  }
+              }
+          }.to_json)
+          .to_return(headers: {content_type: "application/vnd.api+json"}, body: {
+              errors: [
+                  {
+                      id: "1234-abcd",
+                      status: "400",
+                      code: "1337",
+                      title: "bar is required",
+                      detail: "bar include is required for creation",
+                      source: {
+                          parameter: "include"
+                      }
+                  }
+              ]
+          }.to_json)
+
+      article = CustomErrorArticle.create({
+                                   title: "Rails is Omakase",
+                                   email_address: "bar"
+                               })
+
+      assert_equal ["bar include is required for creation"], article.errors[:base]
+    end
+  end
+
   def test_can_handle_explicit_null_error_values
       stub_request(:post, "http://example.com/articles")
         .with(headers: {content_type: "application/vnd.api+json", accept: "application/vnd.api+json"}, body: {