Command line log abstraction

Fixes #1142

I've added two functions to Utils.cpp; `logInfo()` and `logError()`. Both work like `printf()`, taking a format C-string and a variable list of arguments. If we've built with SDL 2, they will use `SDL_LogMessageV()`. Otherwise, they will use a combination of `printf()`and `vprintf()`.

I've also tried to include class names in all of our error messages. This should help us hunt things down when there's a problem.