Use more idiomatic go for errors + ranging
Use more idiomatic go for errors + ranging

* replace fmt.Errorf with errors.New where appropriate
* Delete(keyID) will remove key from keys rather than create new slice
  of keys
* More idiomatic and clear ranging over keys to change status, etc.

google/tink,google/tink,google/tink,google/tink,google/tink,google/tink,google/tink,google/tink