Note that you can create a location user with this call, but not a location admin user. You need to create a location admin user either while you are first adding the location, or, after the fact, through a location update call.
This call, and the other POST calls, will set the member ID in the UserResult structure to a negative number on failure. Here are the values related to this method:
-1: invalid credentials (API key and/or client ID)
-2: missing field (username, password, or role)
-3: related ID missing or invalid
-4: invalid role
-5: exception while creating user (see errorMsg for details)