This PR updates the guidance we have for errors to reflect best practices since
Go 1.13's error APIs. The high-level overview of the guidance is:

- use `errors.New` for static strings,
  exporting a `var` if it needs to be matched
- declare a custom error type if it needs to be matched
- use `fmt.Errorf` for dynamic strings if it does not need to be matched
- use `fmt.Errorf` with `%w` when wrapping if the wrapped error needs to be
  matched
- name top-level error variables `errFoo` or `ErrFoo`
- name error types `FooError`

Resolves #132 #8 
Ref GO-1014