Return correct HTTP status for failure to create an application through /rest/appmanagement/1/application

When creating a new application through rest/appmanagement/1/application fails the internal error is reported as a 500. This should be a 409 Conflict so clients can distinguish it from a server failure and deal with this case.

When picking up this issue, make sure you have a look at the following comment since we need to make sure what flavour of 40x we need to return.