Error if product type is not selected

[sydneywait]

Description

I've included an error message in a required field to throw an error if a product type is not selected

Related Ticket(s)

10 -- https://trello.com/c/FFMVTk10/10-10-users-can-select-a-product-category-when-selling-a-product

Steps to Test

-- Save, add and commit on your branch before checking out to sw-producttype-select
-- Start the app, log in and click on 'sell a product'
-- add all the information about the product but do not select a product type. It should show an error message below the dropdown box.
-- select the product type and create a product, it should post as normal.

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new errors

[Cluttered04]

🧀

[cfitzgerald9]

🇹🇭🇮🇸 works