User Story 23, Flash Messages for Merchant Create and Update

[memcmahon]

As a visitor
When I am updating or creating a new merchant
If I try to submit the form with incomplete information
I see a flash message indicating which field(s) I am missing

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 tested in development
• code is reviewed
• branch is merged
• all tests still pass

child of #12

child of #37