Settings form requires a nonexistent permission

Problem/Motivation

The settings form requires the permission access site administration, but that permission doesn't exist. So only administrators can open the form.

It was incorrectly changed to this in #3317067: Wrong permission used for config form. The core persmission it should use is administer site configuration.

Steps to reproduce

1. Log in as a user who has administer site configuration but isn't an administrator.
2. Go to /admin/config/system/customerror.
3. You get access denied.

Proposed resolution

• Use administer site configuration, same as core's settings forms.
• Add a test