doc.d: move file writes to caller

[WalterBright]

By doing this:

1. gendocfile is relieved of any need for dealing with files and can concentrate on just generating the ddoc text
2. the buffer memory can be recycled instead of free'd and alloc'd again
3. the error handling is now the caller's problem
4. doc.d doesn't need to import errors.d anymore
5. the caller can now decide what to do with the generated output

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#15535"

[WalterBright]

Hmm, I'm going to have to resort to changing this a bit at a time until I find what went wrong.