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
merged changes in /develop
Refactors 403, 404, 500 errors to a single error page 500.html to adhere to DRY principles
updates version requirements in prod settings
moves canvas course site wizard error messages back into the application and out of the containing project ext_tools; also adds subject lines to emails if showing a renderable error
fixes issues brought up during PR 35 review
Merge pull request #35 from Harvard-University-iCommons/feature/TLT-426_custom_error_framework

Feature/tlt 426 custom error framework
merges updates from develop branch, resolves conflict with .gitignore