Changes

Add a tests controller to test the error routes in a consistent and reproducible way.
Remove the error testing routes from application and shows controller
Lump :error and :alert flash messages together, and :notice and :success messages. Mainly useful because :devise sets :alert and :notice.
Replace all instances where :notice flash was set with :success, and all instances where :alert flash was set with :error.
Improve the error template to give more information on an error.

Fixes

Improve error renering so that all flash messages are displayed, rather than just one. It makes a list out of messages of the same type, and puts it in the same alert modal, and consecutively displays them if there are errors of multiple types.