Add error! to rescue_from. Fixes ruby-grape#889.

lib/grape/error_formatter/json.rb

@@ -5,7 +5,7 @@ class << self
         def call(message, backtrace, options = {}, env = nil)
           message = Grape::ErrorFormatter::Base.present(message, env)
 
-          result = message.is_a?(Hash) ? message : { error: message }
+          result = message.is_a?(String) ?  { error: message } : message
           if (options[:rescue_options] || {})[:backtrace] && backtrace && !backtrace.empty?
             result = result.merge(backtrace: backtrace)
           end

lib/grape/middleware/error.rb

@@ -58,6 +58,11 @@ def exec_handler(e, &handler)
         end
       end
 
+      def error!(message, status = options[:default_status], headers = {}, backtrace = [])
+        headers = { 'Content-Type' => content_type }.merge(headers)
+        rack_response(format_message(message, backtrace), status, headers)
+      end
+
       def handle_error(e)
         error_response(message: e.message, backtrace: e.backtrace)
       end

spec/grape/exceptions/validation_errors_spec.rb

@@ -27,7 +27,7 @@ def app
     it 'can return structured json with separate fields' do
       subject.format :json
       subject.rescue_from Grape::Exceptions::ValidationErrors do |e|
-        rack_response e.to_json, 400
+        error!(e, 400)
       end
       subject.params do
         optional :beer