Remove all the panicking Drop implementations

[ljeub-pometry]

Right now all our writers in v4 can panic on drop instead of us properly handling write errors. As there is no way to handle errors in drop properly unfortunately, we will need to replace that by a method that consumes the writer and handles the error.

Alternatively, it might be that we just want to log the errors without panicking as the graph should still be in a sane state at this point (and the write recorded in the WAL if the addition returned successfully).