Informative error messages

[noamross]

Errors such as #13 could have more informative error messages, especially when faterize behavior does diverge from rasterize.

This might be useful in error message writing: http://style.tidyverse.org/error-messages.html

We want to minimize or eliminate overhead of any overhead or assertions, so ideally we do this using error-catching in C++ so that our error code only runs when the error would happen anyway, and not in correctly-executed function calls. (http://gallery.rcpp.org/articles/intro-to-exceptions/)