Can I get Heroku Logs to return only lines outlining errors?

Try this:

heroku logs -t | grep 'error'

To get a running list of errors as and when they occur.

The inverted commas around 'error' did it for me.

heroku logs | grep -i error

You can also install the New Relic add-on, which reports detailed error traces. I've heard good things about Loggly, too.

See also heroku - how to see all the logs

Personally I think that logs aren't the best place to look as the error and the detail are easily missed. I would ensure that my code was raising errors to an external location to ensure that they don't get missed.

There's a number of options ranging from the simple ExceptionNotifier, to more advanced systems such as Airbrake (which is what I use myself).

Not only will these notify you of the errors, but they'll also given you a stack of meta information that you can use.