[llvm-objdump] --private-headers: change errors to warnings for dynamic section dumping

Fix #54456: `objcopy --only-keep-debug` produces a linked image with invalid
empty dynamic section. llvm-objdump -p currently reports an error which seems
excessive.

```
% llvm-readelf -l a.out
llvm-readelf: warning: 'a.out': no valid dynamic table was found
...
```

Follow the spirit of llvm-readelf -l (D64472) and report a warning instead.
This allows later files to be dumped despite warnings for an input file, and
improves objdump compatibility in that the exit code is now 0 instead of 1.

```
% llvm-objdump -p a.out  # new behavior
...
Program Header:
llvm-objdump: warning: 'a.out': invalid empty dynamic section
% objdump -p a.out
...
Dynamic Section:

```

Reviewed By: jhenderson, raj.khem

Differential Revision: https://reviews.llvm.org/D122505