koa shouldn't special-case ENOENT errors
I noticed that koa special-cases ENOENT errors:
https://github.com/koajs/koa/blob/ed84ee50da8ae3cd08056f944d061e00d06ed87f/lib/context.js#L146-L147
I think this is a mistake: ENOENT errors could happen for a lot of reasons (this happens with google's storage library when the key file isn't found, for example) that shouldn't get a 404 status.
And BTW it's not even documented.