Document JavaScript error handling

We have blocks like this:

        .catch(() => {
          this.isMakingRequest = false;
          Flash('Something went wrong. Please try again.');
        });

The initial discussion is here https://gitlab.slack.com/archives/C0GQHHPGW/p1502104314245483

We need to improve messages and add an ability to log errors to Sentry / somewhere on the server side to be able to check them.

@jschatz1 @filipa @iamphill