Feature/tlt 426 custom error framework

Note: this PR requires the changes with the same feature branch name (feature/TLT-426_custom_error_framework) in icommons_ui and django-canvas-course-site-wizard).

* Adds draft versions of custom display error messages to settings, to be used with subclasses of RenderableExceptions
* Adds icommons_ui logging to settings files
* Updates project-level error page 500.html to show RenderableExceptions if rendered by the custom error page mixin 
* Refactors 403, 404, 500 errors to a single error page 500.html to adhere to DRY principles

Hmm... I was thinking that since default error pages are project-wide settings this should be too, but I think you're right, it could be on a per-app basis. Also, defining a template name (a string, basically) seems like something that should be in a settings file, not hardcoded into a view, and since settings exist at the project level I put it there. Thoughts?