Rust: Add validation support to code generator

mojom/generators/rust/templates/source.tmpl.go

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package templates
 
 const GenerateSourceFile = `
 {{- define "GenerateSourceFile" -}}
 //! This file was auto-generated by the Rust bindings generator.
 #![allow(bad_style)]
 #![allow(unused_imports)]
 #![allow(unused_variables)]
 #![allow(dead_code)]
 
-use mojo::bindings::decoding::Decoder;
+use mojo::bindings::decoding;
+use mojo::bindings::decoding::{Decoder, ValidationError};
 use mojo::bindings::encoding;
 use mojo::bindings::encoding::{Context, DataHeaderValue, DATA_HEADER_SIZE, Encoder};
 use mojo::bindings::message;
 use mojo::bindings::message::MessageHeader;
 use mojo::bindings::mojom;
 use mojo::bindings::mojom::{MojomEncodable, MojomPointer, MojomStruct,
                             MojomUnion, MojomMessage, MojomInterface,
                             MojomInterfaceSend, MojomInterfaceRecv,
                             MojomMessageOption, UNION_SIZE};
 
@@ skipping 24 matching lines @@
 {{template "GenerateEnum" $enum}}
 {{end}}
 
 // Interfaces:
 {{range $interface := .Interfaces -}}
 {{template "GenerateInterface" $interface}}
 {{end}}
 
 {{end}}
 `