Improve error handle

[ohbarye]

Change

1. Make error messages short to keep focusing on error causes.
2. Do not exit on RSpec::Expectations::ExpectationNotMetError that inherits Exception so that pbt continues shrinking phase.

RSpec.describe Pbt do
  it "sorts an array" do
    Pbt.assert do
      Pbt.property(Pbt.array(Pbt.integer)) do |numbers|
        result = sort(numbers)
        result.each_cons(2) do |x, y|
          expect(x).to be <= y
        end
      end
    end
  end
end