made runtests-isolated.py better

Now shows you a neat summary at the end and exits with a non-zero exitcode on failure.

Example output:

```
Result: FAIL
19 Failures:
- CheckTests.test_no_sekizai
- ViewPermissionTests.test_unauthed_num_queries
- ViewPermissionTests.test_public_for_all
- ViewPermissionTests.test_page_permissions_view_groups
- ViewPermissionTests.test_page_permissions
- ViewPermissionTests.test_public_for_all_staff_assert_num_queries
- ViewPermissionTests.test_authed_basic_perm_num_queries
- ViewPermissionTests.test_unauthed_no_access
- ViewPermissionTests.test_global_permission
- ViewPermissionTests.test_unauthed
- ViewPermissionTests.test_public_for_all_num_queries
- ViewPermissionTests.test_authed_no_access
- ViewPermissionTests.test_public_for_all_staff
- ViewPermissionTests.test_unauthed_no_access_num_queries
- ViewPermissionTests.test_authed_basic_perm
- PlaceholderTestCase.test_placeholder_tag_language
- PlaceholderTestCase.test_placeholder_tag
- PublishingTests.test_publish_home
- StaticFilesTest.test_collectstatic_with_cached_static_files_storage
```