discourse/app/views/users/omniauth_callbacks/failure.html.erb

<div id="simple-container">
  <%if flash[:error].present? %>
    <div class='alert alert-error'>
      <%=flash[:error]%>
    </div>
  <%else%>
    <div class='alert alert-error'>
      <%= t 'login.omniauth_error_unknown' %>
    </div>
  <%end%>
</div>