closes #2545

What this PR does

• Realign all the error pages
• Refactors error pages so they all inherit from 1 base file, now called error-base.html (previously was error.html). Now both Admin and Benefits have an error-base. The base naming is used like this now:
  , to let the developer know that this is a template that inherits from base.html, and other templates then inherit it.
• error-base.html now makes it so that there is one place to make these changes, making it easier to review also. When making a new error page that is based on this template/design, the developer should not have to write any padding/margin/centering/alignment classes or code. They should only have to write copy (headline-text), create a new or use an existing includes for an icon, add any <p> of text, and use an existing button, or create a new one (the button might have existing CSS classes, like login). But there shouldn't be a need for any new rows or justify-content-centers or col-lg-6. That is the responsibility of the error-base template.

How error-base works:

{% extends "core/base.html" %}  /* inherits from base, so any blocks in base can be used - even if not specified here */
{% load i18n %}

{% block page-title %}
  {% block title %}
  {% endblock title %}
{% endblock page-title %}

/* inherits from base, so you could put a block for nav-buttons here to have a nice area to put the log out button */

{% block inner-content %}   /* inherits from base, `inner-content` has a `<div class="row justify-content-center`"> outside of it, so we can omit it. you have 2 options here: override inner-content entirely, OR, onlly override the children: icon, headline-text, paragraphs. all of our error pages now do this, but in the future if there is some weird new one-off error design where these elements are in a different order, we can override inner-content directly*/
  <div class="col-lg-6">
    <div class="mt-5 pt-4 text-center">
      {% block icon %} /* put the icon includes in here directly, no wrapper div needed */
      {% endblock icon %} /* Default: sadbus icon. If sadbus, then don't need to include this block at all */
    </div>
    <h1 class="h2 py-4">
      {% block headline-text %} /* put the text in here directly, no wrapper div needed */
      {% endblock headline-text %}
    </h1>
    {% block paragraphs %} /* put paragraph elements and other includes (like agency-links) in here directly, no wrapper div needed */
    {% endblock paragraphs %}
  </div>
{% endblock inner-content %}

{% block call-to-action-button %} /* the logic here is, "show the button to home page by default if a template doesn't specify one. or override the button by putting in your own block. */
  {% include "core/includes/button--origin.html" %}
{% endblock call-to-action-button %}

Error pages

1. 500
2. 404
3. 400
4. 200 user error
5. OAuth system error
6. Enrollment system error
7. Enrollment retry
8. Re-enrollment CalFresh
9. Eligibility unverified

Side note outside of the scope of this PR/Milestone

• The transit agency name is supposed to be bold on the agency-links, but it's not. Not part of this PR.
• I noticed an inconsistency in button text casing. There's Start Here and Return home and Try again. Not part of this PR.
• Some files are set to 4 space tabs, while most are 2: reenrollment-error--calfresh and start--medicare at least. Not sure why. Not part of this PR.
• Testing these pages is kinda cumbersome. Perhaps we could have some sort of helper file / method that makes it easier to access all of them.