Add test condition for repository_lib.create_tuf_client_directory().

Verify that non-OSError.EEXist exceptions are re-raised.
Add test coverage for repository_lib._log_status_of_top_level_roles().

Verify that status() does not raise 'tuf.InsufficientKeysError' nor 'tuf.UnsignedMetadataError'.
Merge pull request #229 from vladimir-v-diaz/develop

Improve test coverage for repository_lib.py