FR: provide context for warnings

[fmichonneau]

Currently, any warnings generated during the build process are gathered and displayed at the end. However (and especially when using remake for complex projects with many targets), it can be challenging to identify which stage of the process is generating warnings. It would be great if there was a way to capture the targets associated with these warnings to identify which parts of the code may be causing issues.

[wlandau]

As a remake developer, maybe you could insert 'withr::with_options(list(warn = 1), {...})', but this would override the wishes of users who want to treat warnings as errors ('warn = 2'). As a user, maybe just manually call 'options(warn = 1)'?

[krlmlr]

I like how rmonad handles this. There, each computation returns a result and full context (output, errors, warnings, ...). Errors propagate.