Make base:ReplaceFile return an informative error.

base/platform_file.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.
 
 #ifndef BASE_PLATFORM_FILE_H_
 #define BASE_PLATFORM_FILE_H_
 
 #include "build/build_config.h"
 #if defined(OS_WIN)
 #include <windows.h>
 #endif
 
 #include <string>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/files/file_path.h"
 #include "base/time.h"
 
 namespace base {
 
-#if defined(OS_WIN)
-typedef HANDLE PlatformFile;
-const PlatformFile kInvalidPlatformFileValue = INVALID_HANDLE_VALUE;
-#elif defined(OS_POSIX)
-typedef int PlatformFile;
-const PlatformFile kInvalidPlatformFileValue = -1;
-#endif
-
 // PLATFORM_FILE_(OPEN|CREATE).* are mutually exclusive. You should specify
 // exactly one of the five (possibly combining with other flags) when opening
 // or creating a file.
 enum PlatformFileFlags {
   PLATFORM_FILE_OPEN = 1 << 0,             // Opens a file, only if it exists.
   PLATFORM_FILE_CREATE = 1 << 1,           // Creates a new file, only if it
                                            // does not already exist.
   PLATFORM_FILE_OPEN_ALWAYS = 1 << 2,      // May create a new file.
   PLATFORM_FILE_CREATE_ALWAYS = 1 << 3,    // May overwrite an old file.
   PLATFORM_FILE_OPEN_TRUNCATED = 1 << 4,   // Opens a file and truncates it,
@@ skipping 72 matching lines @@
   // The last modified time of a file.
   base::Time last_modified;
 
   // The last accessed time of a file.
   base::Time last_accessed;
 
   // The creation time of a file.
   base::Time creation_time;
 };
 
+#if defined(OS_WIN)
+typedef HANDLE PlatformFile;
+const PlatformFile kInvalidPlatformFileValue = INVALID_HANDLE_VALUE;
+PlatformFileError LastErrorToPlatformFileError(DWORD saved_errno);
+#elif defined(OS_POSIX)
+typedef int PlatformFile;
+const PlatformFile kInvalidPlatformFileValue = -1;
+PlatformFileError ErrnoToPlatformFileError(int saved_errno);
+#endif
+
 // Creates or opens the given file. If |created| is provided, it will be set to
 // true if a new file was created [or an old one truncated to zero length to
 // simulate a new file, which can happen with PLATFORM_FILE_CREATE_ALWAYS], and
 // false otherwise.  |error| can be NULL.
 //
 // This function fails with 'access denied' if the |name| contains path
 // traversal ('..') components.
 BASE_EXPORT PlatformFile CreatePlatformFile(const FilePath& name,
                                             int flags,
                                             bool* created,
@@ skipping 106 matching lines @@
     return temp;
   }
 
  private:
   PlatformFile* value_;
 };
 
 }  // namespace base
 
 #endif  // BASE_PLATFORM_FILE_H_