gofmt -s and golint

I'm trying to wrap my head around fleet, and I'm going to try to clean up some of the code as I go through. I did a first pass with gofmt/govet/golint. Changes here:

* omitting type annotations for literals when inferrable from context (`gofmt -s`)
* omitting second `range` key when ignored (`k, _` -> `k`, `gofmt -s`)
* Idiomatic error messages: Start lowecase, except where referencing an uppercase value/type, no terminal punctuation (`golint`)
* `errors.New(fmt.Sprintf(args))` -> `fmt.Errorf(args)` (`golint`)
* `if foo { return bar } else { baz }` -> `if foo { return bar } baz` (`golint`)

I guess the error message changes imply a bit of a functionality change. I can revert those if you don't want them to change.