I have a trigger that fires when an opportunity is updated, as part of that I need to call our API with some detail from the opportunity.

As per many suggestions on the web I've created a class that contains a @future method to make the callout.

I'm trying to catch an exception that gets thrown in the @future method, but the test method isn't seeing it.

The class under test looks like this:

public with sharing class WDAPIInterface {
    public WDAPIInterface() {

    }

    @future(callout=true) public static void send(String endpoint, String method, String body)  {
        HttpRequest req = new HttpRequest();
        req.setEndpoint(endpoint);
        req.setMethod(method);
        req.setBody(body);

        Http http = new Http();
        HttpResponse response = http.send(req);

        if(response.getStatusCode() != 201) {
            System.debug('Unexpected response from web service, expecte response status status 201 but got ' + response.getStatusCode());
            throw new WDAPIException('Unexpected response from web service, expecte response status status 201 but got ' + response.getStatusCode());
        }
    }
}

here's the unit test:

@isTest static void test_exception_is_thrown_on_unexpected_response() {
        try {
            WDHttpCalloutMock mockResponse = new WDHttpCalloutMock(500, 'Complete', '', null);
            WDAPIInterface.send('https://example.com', 'POST', '{}');
        } catch (WDAPIException ex) {
            return;
        }

        System.assert(false, 'Expected WDAPIException to be thrown, but it wasnt');
    }

Now, I've read that the way to test @future methods is to surround the call with Test.startTest() & Test.endTest(), however when I do that I get another error:

METHOD RESULT 
test_exception_is_thrown_on_unexpected_response : Skip

 MESSAGE 
Methods defined as TestMethod do not support Web service callouts, test skipped

So the question is, how do I unit test a @future method that makes an callout?