Halt on Ddoc errors & fix the existing ones

[wilzbach]

Required for dlang/dmd#7440

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[andralex]

I think all you need to set the error level is grep -v "Warning:".

[andralex]

Also a problem with this pattern is that if dmd fails with no warning the command will succeed.

[wilzbach]

Okay I removed the explicit grepping for error messages and rebased the PR to just contain the Ddoc fix so that it's absolutely non-controversial and we can move on with this, because once this is merged we can merge the dlang/dmd#7440 at DMD and then -w will work as expected with a proper exit code and no grepping is required.