Remove custom warning and error headings

Problem/Motivation

The current method for allowing site admins to customize the headings used for warnings and errors is helpful, but it gets in the way of having that translated as we prep this module to support multiple languages.

1. The heading settings should be removed and converted to hard coded strings that are properly run through the t().
2. Other subheadings from the template should also be translated.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes