`observeFragment` should have proper way to handle query errors

Currently, `observeFragment` does nothing about query errors, as it does not notify the observer at all when an error occurs. While I also think that it's inappropriate to return network errors from fragment observation, I believe it should fallback to returning missing data at least.

Start at the observeFragment entry point and trace how query errors and observer notifications are handled. Compare the existing missing-data behavior and relevant observation tests, if present. Done means a query error produces the agreed missing-data result for the observer rather than leaving it uninformed.

Written by the indexing model from the issue text.