Fix for an NPE if a location was just forced
Unused imports in MainActivity
Kickass error handling for weather data

Before, error handling was relatively… poor and was pretty much
returning null if anything bad happened. Thanks to a new WeatherResult
class that wraps around WeatherData it's easy for the calling code to
see how things went. If things did not go so smoothly a good bit of
error information is available that can be reported to the user or
logged.
Drastically shortened convertYahooCode switch