Fix error message when deleting an entry referenced in another vocabulary

Currently when a user tries to delete an entry which is referenced in another vocabulary, this error is displayed

ERROR: An error occurred while deleting this entry.

which is the same message as if any other problem occurred during the deletion.

The code to display the error message is

            if (err.status === 401) {
              this.fire('notify', {message: this.i18n('label.error').toUpperCase() + ': ' +
                                            this.i18n('vocabularyManagement.cannotDelete.referencedEntry')});
            } else {
              this.fire('notify', {message: this.i18n('label.error').toUpperCase() + ': ' +
                                            this.i18n('vocabularyManagement.cannotDelete.error')});
            }

so the expected message could be displayed if a status coede 401 was returned but:
1) this status code is never returned
2) 401 is not relevant with this type of error

Given the fact that the correct error message is part of the HTTP response

{"entity-type":"exception","status":400,"message":"This entry is referenced in another vocabulary."}

it should be possible to display the appropriate message by testing the "message" variable.

Even if the message in the UI would change when this request will be taken into account, I don't think it will bother a lot of people and cause regressions in their functional tests.