microsoft/compiler: misc error-handling fixes

While vetting !12510 (merged), I found a load of problems with our current error-handling.

Just the size of the last commit makes me worry if we can actually ever get error-returning right with any level of confidence.

The alternative would be to use setjmp / longjmp to implement something that looks a bit more like exception handling. My biggest worry with that is that the inner parts of that would have to span multiple levels of abstractions, because we allocate memory both in the nir-to-dxil, but also in in the dxil_module helper etc. Thoughts?