capturing error message from echo in cmd prompt

I'm writting out some text to a text file within a cmd batch script like such:

echo FlagValue=Y>>flag.txt

This normally works fine but occassionally if the text file is open by a different process, an error messgae is returned saying Access Denied. What I'd like to do is stop the batch file if an error occurs with something like:

if return_code GEQ 1 GOTO ERR

But can't find a return code fr开发者_如何学编程om echo command. Does one exist, or is there a better tactic to use to capture error message?

echo FlagValue=Y>>flag.txt || echo access_denied Ensure you have rights

or

echo FlagValue=Y>>flag.txt
if /i %errorlevel% NEQ 0 do (
  echo access_denied Ensure you have rights
  call sendmail.cmd
)

Sample:

C:\Users\Me\Desktop>echo Hello > MyFile.txt || echo ERROR
Access is denied.
ERROR

C:\Users\Me\Desktop>echo Hello > a.txt || echo ERROR

C:\Users\Me\Desktop>

Everytime you run a command the ERRORLEVEL environment variable is set to your command's return. So try echo %ERRORLEVEL% straight after you run your command. (Be careful as any command you run inbetween (including echo) will override the %ERRORLEVEL%.

Also, check these out for more information:
Can a batch file capture the exit codes of the commands it is invoking?
Batch Files - Error Handling