I'm in the process of making `LoadScriptingResources` interactively ask
a user whether to load a script. I'd like to turn the existing warning
into the prompt. The simplest way to achieve this is to not print into a
`feedback_stream` parameter, and instead create a prompt right there.
This patch removes the `feedback_stream` parameter and emits a
`ReportWarning` instead. If we get around to adding the prompt instead
of the warning, those changes will be simpler to review. But even if we
don't end up replacing the warning with a prompt, moving away from
output parameters and towards more structured error reporting is a
nice-to-have (e.g., the `warning` prefix is now colored, IDEs have more
flexibility on how to present the warning, etc.).
For a command-line user nothing should change with this patch (apart
from `warning:` being highlighted).
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