Improve error handling in controllers

[markspolakovs]

This PR makes various changes related to error handling in the HTTP controllers, most of which fall into one of two changes:

1. Instead of either just logging the error and returning a blank screen or rendering a 404, delegate all controller error handling to Controllers.handleError, which takes care of logging it (with context), as well as rendering an appropriate template. (In the process this also introduces a 500 template.)
2. Remove the error return from utils.RenderTemplate, since all callers were just doing a log.Println(err); return anyway. Instead have utils take care of logging it, with appropriate context.

Depends on UniversityRadioYork/myradio-go#64.

Fixes #180.