[llvm] [BOLT] Properly propagate Cursor errors (PR #84378)

[Maksim Panchenko via llvm-commits]

maksfb wrote:

> General suggestion: you can set and propagate errors from DE, e.g. `DE.getULEB128(&Offset, &Err)`

`Cursor` interface is an alternative to passing `Offset` and `Err` to `DE` functions.

https://github.com/llvm/llvm-project/pull/84378