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
Never use `validates_...` methods, they're old. Pass `length: { minimum: 10, maximum: 60 }` to the `validates` method instead.

More documentation: http://guides.rubyonrails.org/active_record_validations.html#length