GN: Make file/dir resolving return errors.

tools/gn/function_write_file.cc

 // Copyright (c) 2013 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.
 
 #include <iostream>
 #include <sstream>
 
 #include "base/files/file_util.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
@@ skipping 82 matching lines @@
                    const FunctionCallNode* function,
                    const std::vector<Value>& args,
                    Err* err) {
   if (args.size() != 2) {
     *err = Err(function->function(), "Wrong number of arguments to write_file",
                "I expected two arguments.");
     return Value();
   }
 
   // Compute the file name and make sure it's in the output dir.
-  if (!args[0].VerifyTypeIs(Value::STRING, err))
+  const SourceDir& cur_dir = scope->GetSourceDir();
+  SourceFile source_file = cur_dir.ResolveRelativeFile(args[0], err,
+      scope->settings()->build_settings()->root_path_utf8());
+  if (err->has_error())
     return Value();
-  const SourceDir& cur_dir = scope->GetSourceDir();
-  SourceFile source_file = cur_dir.ResolveRelativeFile(args[0].string_value(),
-      scope->settings()->build_settings()->root_path_utf8());
   if (!EnsureStringIsInOutputDir(
           scope->settings()->build_settings()->build_dir(),
           source_file.value(), args[0].origin(), err))
     return Value();
 
   // Compute output.
   std::ostringstream contents;
   if (args[1].type() == Value::LIST) {
     const std::vector<Value>& list = args[1].list_value();
     for (const auto& cur : list)
@@ skipping 22 matching lines @@
   if (DoWriteFile(file_path, new_contents.c_str(), int_size)
       != int_size) {
     *err = Err(function->function(), "Unable to write file.",
                "I was writing \"" + FilePathToUTF8(file_path) + "\".");
     return Value();
   }
   return Value();
 }
 
 }  // namespace functions