Add method to `WebHook` to explain state

See discussion:

#343993 (comment 727053383)

From looking at the original UX issue, we have 3 scenarios and I expect us to have a helper / model that looks like this:

if hook.expected_error?
  # Scenario 1
elsif hook.unexpected_error?
  # Scenario 2
elsif hook.rate_limit_error?
  # Scenario 3
end