login_page.rb

require 'watir'

class LoginPage
  def initialize browser, url = 'https://www.linkedin.com/uas/login'
    @browser, @url = browser, url
  end

  def sign_in_with login, password
    @browser.goto @url
    @browser.input(id: 'session_key-login').send_keys login
    @browser.input(id: 'session_password-login').send_keys password
    @browser.input(id: 'btn-primary').click
  end

  def error_occurred?
    @browser.text.include? "There were one or more errors in your submission."
  end

  def logged_in?
    @browser.url == 'https://www.linkedin.com/feed/?trk='
  end
end

login_page = LoginPage.new Watir::Browser.new
login_page.sign_in_with 'nonexistent_email@some.com', 'some password'
login_page.error_occurred? #=> true
login_page.logged_in?      #=> false