User Story 49, Merchant cannot add an item if details are bad/missing

[iandouglas]

As a merchant
When I try to add a new item
If any of my data is incorrect or missing (except image)
Then I am returned to the form
I see one or more flash messages indicating each error I caused
All fields are re-populated with my previous data

DO THESE STORIES FIRST:

• User Story 48, Merchant adds an item

Checklist:

• base tests are written
• base code is written to pass tests
• edge case ("sad path") tests written (if required)
• edge case code written (if required)
• code is reviewed
• branch is merged
• all tests still pass

child of #72

child of #125

depends on #67