Add config to customize the default error message

Add a lowlevel_error_handler, so we can customize the default error
message.

example:

```
lowlevel_error_handler do
  [302, {'Content-Type' => 'text', 'Location' => 'foo.html'}, ['302 found']]
end
```

[fix #458]

lib/puma/configuration.rb

@@ -327,6 +327,15 @@ module Puma
       def preload_app!(answer=true)
         @options[:preload_app] = answer
       end
+
+      # Use +obj+ or +block+ as the low lever error handler. This allows a config file to
+      # change the default error on the server.
+      #
+      def lowlevel_error_handler(obj=nil, &block)
+        obj ||= block
+        raise "Provide either a #call'able or a block" unless obj
+        @options[:lowlevel_error_handler] = obj
+      end
     end
   end
 end

lib/puma/server.rb

@@ -707,6 +707,10 @@ module Puma
     # A fallback rack response if +@app+ raises as exception.
     #
     def lowlevel_error(e)
+      if handler = @options[:lowlevel_error_handler]
+        return handler.call(e)
+      end
+
       if @leak_stack_on_error
         [500, {}, ["Puma caught this error: #{e.message} (#{e.class})\n#{e.backtrace.join("\n")}"]]
       else

test/config/app.rb

@@ -1,3 +1,7 @@
 app do |env|
   [200, {}, ["embedded app"]]
 end
+
+lowlevel_error_handler do |err|
+  [200, {}, ["error page"]]
+end

test/test_config.rb

@@ -13,4 +13,14 @@ class TestConfigFile < Test::Unit::TestCase
 
     assert_equal [200, {}, ["embedded app"]], app.call({})
   end
+
+  def test_lowleve_error_handler_DSL
+    opts = { :config_file => "test/config/app.rb" }
+    conf = Puma::Configuration.new opts
+    conf.load
+
+    app = conf.options[:lowlevel_error_handler]
+
+    assert_equal [200, {}, ["error page"]], app.call({})
+  end
 end

test/test_puma_server.rb

@@ -220,6 +220,23 @@ class TestPumaServer < Test::Unit::TestCase
     data = sock.read
 
     assert_not_match(/don't leak me bro/, data)
+    assert_match(/HTTP\/1.0 500 Internal Server Error/, data)
+  end
+
+  def test_prints_custom_error
+    @events = Puma::Events.strings
+    re = lambda { |err| [302, {'Content-Type' => 'text', 'Location' => 'foo.html'}, ['302 found']] }
+    @server = Puma::Server.new @app, @events, {lowlevel_error_handler: re}
+
+    @server.app = proc { |e| raise "don't leak me bro" }
+    @server.add_tcp_listener @host, @port
+    @server.run
+
+    sock = TCPSocket.new @host, @port
+    sock << "GET / HTTP/1.0\r\n\r\n"
+
+    data = sock.read
+    assert_match(/HTTP\/1.0 302 Found/, data)
   end
 
   def test_custom_http_codes_10