- the interactive menus didn't allow interactive users to select invalid combinations...
- ... but if specified directly on cmdline, no checks were done at all.
- introduce `exit_with_target_not_supported_error()`, which is just `exit_with_error` (code 43) but with code 44
- this way the targets pipeline can just warn instead of break;
- sometimes it's easier multiplying matrixes and skipping the few that can't be built
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