We should expose the actual line from the settings that caused a warning

From #11225 originally.

> It would be helpful if the error message showed either the actions or the keybindings causing the error. Otherwise the user has to guess.

We do this already for some types of serialization error, but not all. Maybe we could expose the action name? Or the line that the action is on, to at least point them in the right direction.

Start by tracing the settings serialization warnings and compare the error types that already expose action or keybinding information with those that do not. Done means the relevant warning identifies the action, keybinding, or settings line that caused it, rather than requiring users to guess.

Written by the indexing model from the issue text.