make `-w dupbuild` default to `err`

You can still opt out of this by passing `-w dupbuild=warn`.
But if you're getting this diagnostic, your build files are incorrect
and you should ideally just fix them.

This is step 3 on https://github.com/ninja-build/ninja/issues/931
I sent an RfC to ninja-build a few months ago; nobody objected.
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