You can re-direct error output by using its descriptor, which is '2'. For example, using the find command from the root directory as a user generates errors because users don't have permissions to see all of the directories on the file system. You can re-direct the error messages to /tmp/output.log with:
find / -name dummyfile 2>/tmp/output.log
Or, just stop the errors displaying with:
find / -name dummyfile 2>/dev/null
If you're using bash as your shell, type man bash
and search for redirection.