Assert used instead of if

[fabianegli]

The same issue as in pyensembl issues 227 - assert is removed form optimized code and should be reserved for testing.

$ git grep assert datacache
datacache/cache.py:        assert subdir
datacache/common.py:    assert download_url or filename, "Either filename or URL must be specified"
datacache/database_table.py:        assert len(key_set) == len(key_list), \
datacache/download.py:            assert len(names) > 0, "Empty zip archive"
datacache/download.py:    assert os.path.exists(transformed_path)