[llvm-readobj] - Simplify the code that creates dumpers. NFCI.

We have a few helper functions like the following:
```
std::error_code create*Dumper(...)
```

In fact we do not need or want to use `std::error_code` and the code
can be simpler if we just return `std::unique_ptr<ObjDumper>`.

This patch does this change and refines the signature of `createDumper`
as well.

Differential revision: https://reviews.llvm.org/D86718