Network error handling

I've only found a few bugs and fixed a few in the past couple of days but one of the fixes was a big one. I now have a unified way of handling network errors within the application so that, whether they come from a problem with a connection or are returned as server text, they propagate through the application from the network layer to the top-level callers in the same way.

(Debugging asynchronous networking is the same kind of fun as swinging blindly at spontaneously-regenerating, rocket-powered pinatas.)

Bug score: Open 35, Fixed 11.