Refactor CSharpExporter for improved error handling

[larpinguser]

issues fixed:
-redundant decompiler null checks
-export_resource crashes instead of reporting
-empty get_handled_importers unexplained
-duplicated "cs" string literal
-misleading early err = OK declaration
-no input validation before expensive decompile

[nikitalita]

Thanks!

Just curious, was this to fix bugs that you ran into, or was this just general clean-up?

[AITYunivers]

Thanks!

Just curious, was this to fix bugs that you ran into, or was this just general clean-up?

It's AI, don't merge this. He did the same to my repo.

[larpinguser]

there was basically no ai usage for this one, (if i knew you would come here to talk about this pr i would have recorded me manually typing the code)
2. this was a general clean up, just picked a file and fixed some stuff with it.

[larpinguser]

^^ closed because I didn't see a real purpose in this pr, although I supposed ill make a patch release someday