[llvm] [BOLT] Exit with error code on missing DWO CU (PR #125976)

[Maksim Panchenko via llvm-commits]

https://github.com/maksfb approved this pull request.

Add a brief summary why you are converting the assertion. Small nit for the error message. Otherwise LGTM.

https://github.com/llvm/llvm-project/pull/125976