Note that I said "I would use cron to schedule it to run every five minutes". If an error occurs no more than 5 minutes later, cron will run the script. I think that an average wait of 2.5 minutes...
I would write a script to examine errlog and if finds something, it will email someone. It would do this once, it would not loop. Then I would use cron to schedule it to run every five minutes.
What error message? The best you can reasonably do is to check for the error, say, once every 5 minutes or so. You can check as often as you like within reason, but you don't want to overload the...