osxkeychain: Delete(): return typed errors

[thaJeztah]

This allows a Delete for non-existing credentials to be handled.

[codecov-commenter]

Codecov Report

Patch coverage: 36.84% and project coverage change: -0.32 :warning:

Comparison is base (9ff5b61) 55.55% compared to head (b21b69c) 55.23%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #278 +/- ## ========================================== - Coverage 55.55% 55.23% -0.32% ========================================== Files 9 9 Lines 666 668 +2 ========================================== - Hits 370 369 -1 - Misses 253 256 +3 Partials 43 43 ``` | [Impacted Files](https://app.codecov.io/gh/docker/docker-credential-helpers/pull/278?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=docker) | Coverage Δ | | |---|---|---| | [osxkeychain/osxkeychain\_darwin.go](https://app.codecov.io/gh/docker/docker-credential-helpers/pull/278?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=docker#diff-b3N4a2V5Y2hhaW4vb3N4a2V5Y2hhaW5fZGFyd2luLmdv) | `69.10% <36.84%> (-1.97%)` | :arrow_down: |

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[thaJeztah]

@crazy-max ptal 🤗

[thaJeztah]

@crazy-max updated; PTAL