prime a mock call to return an error and test for it

test_error.go

Context("simulate an error retrieving data from S3", func() {
        var testCall *gomock.Call
        BeforeEach(func() {
                testCall = mockFetcher.EXPECT().GetObject(gomock.Any()).Return(nil, fmt.Errorf("AWS error")).AnyTimes()
        })
        It("and fails", func() {
                _, err := ReadPersonFromS3(mockFetcher)
                Expect(err).To(HaveOccurred())
                testCall.Times(1)
        })
})