No feedback given to the user when invalid input is provided

[karenetheridge]

e.g. when editing a hardware product field and providing a string where an integer is expected, the POST is sent and then no feedback is given to the user that the input was invalid.

In many cases the check can be done right in the UI (since numbers are submitted properly as numbers in numeric fields, the UI clearly knows which fields are intended to be numeric and which are strings). In other cases the API does additional checks and so the 400 response will need to be detected and conveyed to the user.

(related: #196)