Prompt error message when creating aggregate without aggregate name

When using API to create an availability zone via aggregate, the
prompt message will be "There was a conflict when trying to complete
your request" if the request body looks like below.
{"aggregate": {"name": "", "availability_zone": "zy1_zone53335"}}

The message is not accurate, aggregate name should not be null.

Fixes bug 1188032
Change-Id: I1fb74186886283a9b6763abb665b8ce42e4bb24c
Merge "Prompt error message when creating aggregate without aggregate name"