Truncate number of errors displayed in the web UI

[justiniso]

This change limits the number of rows on the web UI to display at most 500 rows in the stats and failures tabs. A common scenario is the app failing under load, resulting in a large number of unique failures, bringing the rendering to a halt.

A simple locustfile demonstrates the issue:

import random 

from locust import HttpLocust, TaskSet, task

class UserTasks(TaskSet):

    @task
    def error(self):
        self.client.get("/404/{}".format(random.randint(0,99999999999))

class WebsiteUser(HttpLocust):
    min_wait = 0
    max_wait = 0
    task_set = UserTasks

This will generate a large number of results and errors, eventually killing the page. I feel that 500 results is a reasonable number to display as it's more than anyone could consume manually through the web UI. But for automated processes, the actual results will remain intact.

[cgoldberg]

LGTM 👍