If failed to get credential - inform on what input, and use stdout if no stderr

[yarikoptic]

In my case I was trying with such a minimal usecase

import oras.client
from oras.container import Container

client = oras.client.OrasClient()
container = Container("neurodebian:sid")
client.get_manifest(container)

and I had

❯ docker-credential-secretservice list
{"https://index.docker.io/v1/":"repronimservices","workshop.dev.flywheel.io":"yarikoptic@gmail.com"}

Running the script resulted in hard to digest and not very informative traceback which started with

❯ uv run try_oras.py
Credential helper 'docker-credential-secretservice' failed:

NB it then continued with not directly related traceback on JSONDecodeError because was returned HTML I guess instead of json... not clear why continued to start with.

Not yet sure how the "input" should be fixed up (but seems need to be a url not just hostname), but this PR fixes at least the error to become informative. With it we get:

Credential helper 'docker-credential-secretservice' for 'docker.io' failed: credentials not found in native keychain

FTR:

❯ dlocate docker-credential-secretservice
golang-docker-credential-helpers: /usr/bin/docker-credential-secretservice
❯ apt-cache policy golang-docker-credential-helpers | cat
golang-docker-credential-helpers:
  Installed: 0.6.4+ds1-1+b18
  Candidate: 0.6.4+ds1-1+b18
  Version table:
 *** 0.6.4+ds1-1+b18 900
		600 https://deb.debian.org/debian sid/main amd64 Packages
		900 https://deb.debian.org/debian forky/main amd64 Packages
		100 https://deb.debian.org/debian trixie/main amd64 Packages
		100 /var/lib/dpkg/status