remove errors.d from mars.d

[WalterBright]

More elimination of reliance on globals.

Eliminating the calls to fatal() makes it more tractable as a library.

[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#16565"

[WalterBright]

I have no idea what this is about:

ERROR: Found the following residual temporary files.
ERROR: Temporary files should be stored in `test_results` or explicitly removed.
?? compiler/test/test14894a
make: *** [Makefile:120: check-clean-git] Error 1

[thewilsonator]

this is forward declared in LDC, but unused, and not in a header.
fyi @ibuclaw

Does this still need to be extern(C++)?

[WalterBright]

Sigh. Looks like I'll have to do a binary search to find the problem!