Add errcheck linter

This linter ensures that we check the error on functions that return an
error. This will help prevent bugs where we assume that a function such
as Chmod or Open is successful, but forget to check that no error is
returned.

In cases where we do intentionally want to ignore the error, we will now
need to assign it to an empty identifier, such as _ = os.Chmod().

Authored-by: Chris Hajas <chajas@pivotal.io>