[lldb][Process/FreeBSDKernelCore] Improve error handling (#191423)
Improve error handling with the following changes:
- If `kvm_open2()` fails in `DoLoadCore()`, log error with a message
obtained from the function.
- Return false or continue for loop if memory read fails.
- Rename `!error.Success()` to `error.Fail()` for readability (NFC).
Signed-off-by: Minsoo Choo <minsoochoo0122@proton.me>