Entering a password that is too long results in an unhandled exception

# Problem
A password that is too long results in an unhandled exception.

# Steps to Reproduce:
1. In the signup form enter a password that is too long.
2. Click submit.
3. You should see the following page and the following sentry crash report:
[Sentry issue](https://hypothes-is.slack.com/archives/C0LUWQQJJ/p1569548578000100)

# What should happen:
There should be a frontend check that doesn't let the user submit if the password is too long and/or a nice handling of this error that informs the user their password is too long instead of simply crashing.