Fix placement of failure_message blocks

The `failure_message` and `failure_message_when_negated` blocks should be inside `RSpec::Matchers.define` but not inside `match` or they won't have the desired effect.

See also: [examples from RSpec documentation](https://www.relishapp.com/rspec/rspec-expectations/v/3-0/docs/custom-matchers/define-matcher#overriding-the-failure-message).