Do not use ShowExceptions during tests, it confuses things

middleman-core/lib/middleman-core/core_extensions/show_exceptions.rb

@@ -1,14 +1,16 @@
+require 'rack/showexceptions'
+
 # Support rack/showexceptions during development
 module Middleman::CoreExtensions
   class ShowExceptions < ::Middleman::Extension
     def initialize(app, options_hash={}, &block)
       super
 
-      require 'rack/showexceptions'
+      app.config.define_setting :show_exceptions, true, 'Whether to catch and display exceptions'
     end
 
     def after_configuration
-      app.use ::Rack::ShowExceptions
+      app.use ::Rack::ShowExceptions if app.config[:show_exceptions]
     end
   end
 end

middleman-core/lib/middleman-core/step_definitions/server_steps.rb

@@ -41,12 +41,11 @@ Given /^the Server is running$/ do
   ENV['MM_ROOT'] = root_dir
 
   initialize_commands = @initialize_commands || []
+  initialize_commands.unshift lambda { config[:show_exceptions] = false }
 
   @server_inst = Middleman::Application.server.inst do
-    app.initialized do
-      initialize_commands.each do |p|
-        config_context.instance_exec(&p)
-      end
+    initialize_commands.each do |p|
+      instance_exec(&p)
     end
   end