add mvc.Err builtin dependency to map any context/mvc response's.Err field on methods like HandleHTTPError

relative to: https://github.com/kataras/iris/issues/1606

_examples/mvc/error-handler-http/main.go

@@ -1,6 +1,8 @@
 package main
 
 import (
+	"fmt"
+
 	"github.com/kataras/iris/v12"
 	"github.com/kataras/iris/v12/mvc"
 )
@@ -27,6 +29,14 @@ func (c *controller) Get() string {
 	return "Hello!"
 }
 
+func (c *controller) GetError() mvc.Result {
+	return mvc.View{
+		// Map to mvc.Code and mvc.Err respectfully on HandleHTTPError method.
+		Code: iris.StatusBadRequest,
+		Err:  fmt.Errorf("custom error"),
+	}
+}
+
 // The input parameter of mvc.Code is optional but a good practise to follow.
 // You could  register a Context and get its error code through ctx.GetStatusCode().
 //
@@ -39,7 +49,12 @@ func (c *controller) Get() string {
 // a controller's field (or method's input argument) is required
 // to select which, between those two controllers, is responsible
 // to handle http errors.
-func (c *controller) HandleHTTPError(statusCode mvc.Code) mvc.View {
+func (c *controller) HandleHTTPError(statusCode mvc.Code, err mvc.Err) mvc.View {
+	if err != nil {
+		// Do something with that error,
+		// e.g. view.Data = MyPageData{Message: err.Error()}
+	}
+
 	code := int(statusCode) // cast it to int.
 
 	view := mvc.View{

hero/container.go

@@ -168,6 +168,9 @@ var BuiltinDependencies = []*Dependency{
 	NewDependency(func(ctx *context.Context) Code {
 		return Code(ctx.GetStatusCode())
 	}).Explicitly(),
+	NewDependency(func(ctx *context.Context) Err {
+		return Err(ctx.GetErr())
+	}).Explicitly(),
 	// payload and param bindings are dynamically allocated and declared at the end of the `binding` source file.
 }
 

hero/handler.go

@@ -20,7 +20,20 @@ type (
 	ErrorHandlerFunc func(*context.Context, error)
 
 	// Code is a special type for status code.
+	// It's used for a builtin dependency to map the status code given by a previous
+	// method or middleware.
+	// Use a type like that in order to not conflict with any developer-registered
+	// dependencies.
+	// Alternatively: ctx.GetStatusCode().
 	Code int
+
+	// Err is a special type for error stored in mvc responses or context.
+	// It's used for a builtin dependency to map the error given by a previous
+	// method or middleware.
+	// Use a type like that in order to not conflict with any developer-registered
+	// dependencies.
+	// Alternatively: ctx.GetErr().
+	Err error
 )
 
 // HandleError fires when a non-nil error returns from a request-scoped dependency at serve-time or the handler itself.

mvc/aliases.go

@@ -16,6 +16,11 @@ type (
 	// http error handling in controllers.
 	// This can be one of the input parameters of the `Controller.HandleHTTPError`.
 	Code = hero.Code
+	// Err is a type alias for the `hero#Err`.
+	// It is a special type for error stored in mvc responses or context.
+	// It's used for a builtin dependency to map the error given by a previous
+	// method or middleware.
+	Err = hero.Err
 	// DeprecationOptions describes the deprecation headers key-values.
 	// Is a type alias for the `versioning#DeprecationOptions`.
 	//