Use require.Error in test instead of assert.Error to prevent nil poin…

…ter deref

Another issue fixed by a different commit exposed that failure to stop the test
on this error generates a nil pointer dereference when we try to check the
contents of a non-existent error. Using require.Error stops the test before
reaching the dereference.

local_cache/cache_test.go

@@ -330,7 +330,7 @@ func TestClient(t *testing.T) {
 
 		_, err = client.DoGet(ctx, "pelican://"+param.Server_Hostname.GetString()+":"+strconv.Itoa(param.Server_WebPort.GetInt())+"/test/hello_world.txt.1",
 			filepath.Join(tmpDir, "hello_world.txt.1"), false, client.WithToken(token), client.WithCaches(cacheUrl), client.WithAcquireToken(false))
-		assert.Error(t, err)
+		require.Error(t, err)
 		assert.Equal(t, "failed download from local-cache: server returned 404 Not Found", err.Error())
 	})
 	t.Cleanup(func() {