Introduce a WarningsAsErrors block to Robust.Properties.targets

[Mervill]

trafficstars

One way to have fewer warnings is to turn some warnings into errors.

I have one suggestion to start off: CS0105 which is the duplicate using warning. I don't think I care too much about unused usings but duplicate usings should definitely be an error.

It helps take some of the load off of maintainers by turning incidental shit that causes warnings into quick failures. It's not a big ask for either us or downstreams to correct either, so I seems like a no brainer.

[mirrorcult]

wasnt aware you could do this for individual warnings, good idea imo

[Mervill]

The duplicate using from content was removed so if the tests are re-run we should be good.

[PJB3005]

Could this cause undue problems for downstreams when doing merges?