[FEATURE REQUEST] Add [[nodiscard]] to image read/write methods

This is not so much a feature request but more of an open discussion on whether all of the `read_` and `write_` methods should have a `[[nodiscard]]` attribute added (since C++17) to avoid users not catching whether the read/write was successful or not (caught myself doing this).

This would obviously break ABI and backwards compatibility so I'm happy to hear opinions!

Start by surveying the C++ read_ and write_ methods and checking how their return values are currently used. Confirm the C++17, ABI, and backwards-compatibility implications before defining a consistent scope. Done means a maintainer-approved decision and implementation scope; this issue is currently an open discussion rather than a startable task.

Written by the indexing model from the issue text.