Add support for custom error documents and provide an example

error-document.lua

@@ -0,0 +1,12 @@
+return function (app, options)
+	return function (req, res)
+		app(req, function (code, headers, body)
+			local errordocument = options[code]
+			if errordocument then
+				errordocument(code, headers, body, res)
+			else
+				res(code, headers, body)
+			end
+		end)
+	end
+end

errorhandlers.lua

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+local errorhandlers = {}
+
+function errorhandlers.text(text)
+	return function (code, headers, body, res)
+		res(code, headers, text)
+	end
+end
+
+function errorhandlers.file(path)
+	return function (code, headers, body, res)
+		local file = io.open(path)
+		body = file:read("*a")
+		res(code, headers, body)
+	end
+end
+
+function errorhandlers.execute(path)
+	return function (code, headers, body, res)
+		local chunk = loadfile(path)
+		local success, err = pcall(chunk, code, headers, body, res)
+		if not success then
+			error("Failed to execute error document handler: " .. err)
+			res(code, headers, body)
+		end
+	end
+end
+
+function errorhandlers.redirect(url)
+	return function (code, headers, body, res)
+		code = 302
+		headers["Location"] = url
+		res(code, headers, {})
+	end
+end
+
+return errorhandlers

samples/test-errordocument.lua

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+local p = require('utils').prettyPrint
+local socketHandler = require('web').socketHandler
+local createServer = require('uv').createServer
+local errhdl = require('errorhandlers')
+
+local host = os.getenv("IP") or "0.0.0.0"
+local port = os.getenv("PORT") or 8080
+
+local state = 1
+local app = function (req, res)
+  local code = 200
+  if state > 3 then
+    code = 404
+  end
+
+  res(code, {
+    ["Content-Type"] = "text/plain"
+  }, {"Hello ", "World ", tostring(state), "\n"})
+
+  if req.url.path == "/" then
+    state = state + 1
+  end
+end
+
+app = require('error-document')(app, {
+	[404] = errhdl.text("TEST 404"),
+})
+
+app = require('autoheaders')(app)
+
+app = require('log')(app)
+
+p{app=app}
+
+app({
+  method = "GET",
+  url = { path = "/" },
+  headers = {}
+}, p)
+
+createServer(host, port, socketHandler(app))
+print("http server listening at http://localhost:8080/")
+
+require('luv').run()
+
+print("done.")