If the filesystem isn't writable, return a `WP_Error` object instead of lame status array
Use `WP_Error` in a variety of places.
Returning `WP_Error` objects is generally more helpful
Have theme update error reporting follow the same pattern as plugin so we properly report errors from the skin
Merge pull request #98 from humanmade/wp-error-98

Convert use of array error status responses to WP_Error