Make base:ReplaceFile return an informative error.

base/file_util.h

 // Copyright (c) 2012 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.
 
 // This file contains utility functions for dealing with the local
 // filesystem.
 
 #ifndef BASE_FILE_UTIL_H_
 #define BASE_FILE_UTIL_H_
 
@@ skipping 83 matching lines @@
 
 // Same as Move but allows paths with traversal components.
 // Use only with extreme care.
 BASE_EXPORT bool MoveUnsafe(const base::FilePath& from_path,
                             const base::FilePath& to_path);
 
 // Renames file |from_path| to |to_path|. Both paths must be on the same
 // volume, or the function will fail. Destination file will be created
 // if it doesn't exist. Prefer this function over Move when dealing with
 // temporary files. On Windows it preserves attributes of the target file.
-// Returns true on success.
+// Returns true on success, leaving *error unchanged.
+// Returns false on failure and sets *error appropriately, if it is non-NULL.
+BASE_EXPORT bool ReplaceFileAndGetError(const base::FilePath& from_path,
+                                        const base::FilePath& to_path,
+                                        base::PlatformFileError* error);
+
+// Backward-compatible convenience method for the above.
 BASE_EXPORT bool ReplaceFile(const base::FilePath& from_path,
                              const base::FilePath& to_path);
 
 // Copies a single file. Use CopyDirectory to copy directories.
 // This function fails if either path contains traversal components ('..').
 BASE_EXPORT bool CopyFile(const base::FilePath& from_path,
                           const base::FilePath& to_path);
 
 // Same as CopyFile but allows paths with traversal components.
 // Use only with extreme care.
@@ skipping 431 matching lines @@
 
 // Attempts determine the FileSystemType for |path|.
 // Returns false if |path| doesn't exist.
 BASE_EXPORT bool GetFileSystemType(const base::FilePath& path,
                                    FileSystemType* type);
 #endif
 
 }  // namespace file_util
 
 #endif  // BASE_FILE_UTIL_H_