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

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.