Add #[must_use] to WIN32_ERROR and other return types that represent error codes

[kennykerr]

Is it worth marking WIN32_ERROR as #[must_use] @kennykerr? My only concern after upgrading to 0.54.0 is that it is now quite possible to forget to handle errors and no warnings are given about this (unlike previous versions which forced using Result values).

Cheers
Fotis

Originally posted by @fgimian in #2890 (comment)

[kennykerr]

Probably worth applying to WIN32_ERROR, BOOL, NTSTATUS, and RPC_STATUS.

[kennykerr]

This is complicated by the fact that the metadata models some of these as enums and others as structs...