fix: improve logs when Ubuntu Pro credentials are not available

[cristiangirlea]

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Fixes #207

When a release defines Ubuntu Pro archives (as ubuntu-24.04 does) and no
credentials are available, chisel cut logged an error-like message for
every Pro archive followed by a per-archive "ignored" line. Users not
using Pro archives at all found this confusing, and nothing tied the
messages to Ubuntu Pro.

As discussed in #207, Chisel now emits a single informative message when
Pro credentials are not available and stays quiet about it otherwise.
When valid credentials are present the logs are unchanged.

Changes

• internal/archive: a missing credentials directory is treated as the
  normal "no Pro credentials" case and is no longer logged. Any other error
  opening the directory (for example a permission problem) is still logged.
  The doc comment of findCredentialsInDir was also corrected: it claimed
  the function returns empty credentials with a nil error, but it returns
  ErrCredentialsNotFound.
• cmd/chisel: cut and debug check-release-archives collect the archives
  ignored because of missing credentials and report them once, sorted, via a
  small shared helper.
• Tests: TestFindCredentialsInDirMissingDir asserts nothing is logged for a
  missing directory, and a new TestRunIgnoresProArchivesWithoutCredentials
  covers the single-message behaviour through debug check-release-archives
  (which has the archiveOpen hook; cut shares the same logic).

Verification

go test ./... passes and go vet ./... reports nothing new (its only
output is pre-existing unkeyed-field warnings in internal/setup tests).
Both new tests fail on main and pass with this change.

Before / after with a real release, no credentials directory:

CHISEL_AUTH_DIR=/nonexistent chisel cut --release ubuntu-24.04 --root /tmp/rootfs base-files_base

Before:

Cannot open credentials directory "/nonexistent": open /nonexistent: no such file or directory
Archive "ubuntu-esm-infra" ignored: credentials not found
Cannot open credentials directory "/nonexistent": open /nonexistent: no such file or directory
Archive "ubuntu-esm-apps" ignored: credentials not found

After:

Ubuntu Pro subscription not available, ignoring archives: ubuntu-esm-apps, ubuntu-esm-infra

The wording follows the suggestion in #207; happy to adjust it.

Assisted-by: AI tools