Add error message rendering for bad posts

posts#create passes object with error messages back to post upon
validation failure

posts/index renders partial error_messages

error_messages checks if @post object contains errors and display them
if it does
This partial does not have a great name - it renders a form and error messages, not just error messages. If I see `render "error_message"`, I wouldn't assume that it's rendering a form. How about extracting an `error_messages` partial like we did in past projects and using that as part of the form?