GN: Make file/dir resolving return errors.

tools/gn/function_write_file.cc

Index: tools/gn/function_write_file.cc
diff --git a/tools/gn/function_write_file.cc b/tools/gn/function_write_file.cc
index 9f853f87008769695c24495a6df2eff469e0245b..8a6fee31e1c93b01269d99bdf1917c5d2b956369 100644
--- a/tools/gn/function_write_file.cc
+++ b/tools/gn/function_write_file.cc
@@ -100,11 +100,11 @@ Value RunWriteFile(Scope* scope,
   }
 
   // Compute the file name and make sure it's in the output dir.
-  if (!args[0].VerifyTypeIs(Value::STRING, err))
-    return Value();
   const SourceDir& cur_dir = scope->GetSourceDir();
-  SourceFile source_file = cur_dir.ResolveRelativeFile(args[0].string_value(),
+  SourceFile source_file = cur_dir.ResolveRelativeFile(args[0], err,
       scope->settings()->build_settings()->root_path_utf8());
+  if (err->has_error())
+    return Value();
   if (!EnsureStringIsInOutputDir(
           scope->settings()->build_settings()->build_dir(),
           source_file.value(), args[0].origin(), err))