rem http://stackoverflow.com/questions/2593133/batch-file-ren-commands-errorlevel-returns-0-even-on-failure/6969940#6969940
rem Screenshot of the results when having an "Access Denied" error and "The system cannot find the file specified." error when using 'ren' command, both cases DO set the ERRORLEVEL to "1":
rem http://i.imgur.com/BQb2Ydg.png

@echo OFF

SET directory=d:\downloads\testdir

SET rename_what_filename=bla.txt
SET rename_what_path=%directory%\%rename_what_filename%
SET rename_to=sad.txt
echo Renaming "%rename_what_filename%" to "%rename_to%" in the following directory: "%directory%"
ren %rename_what_path% %rename_to%

if not %errorlevel%==0 (
  echo There was a problem renaming the file.
) else (
  echo Renaming was successful.
)

echo Errorlevel: "%errorlevel%"

echo.

SET rename_what_filename=test.txt
SET rename_what_path=%directory%\%rename_what_filename%
SET rename_to=asdsad.txt
echo Renaming "%rename_what_filename%" to "%rename_to%" in the following directory: "%directory%"
ren %rename_what_path% %rename_to%

if not %errorlevel%==0 (
  echo There was a problem renaming the file.
) else (
  echo Renaming was successful.
)

echo Errorlevel: "%errorlevel%"