credentials: improve errors and error-handling

[thaJeztah]

credentials: use errors.Is() to match error-types

credentials: implement errdefs types for typed errors

This allows for checking the error-type returned to be matched with the errdefs utilities.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.

Project coverage is 52.51%. Comparing base (bd83e02) to head (a228f55). Report is 42 commits behind head on master.

Files	Patch %	Lines
credentials/error.go	44.44%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #292 +/- ## ========================================== - Coverage 52.74% 52.51% -0.24% ========================================== Files 9 9 Lines 673 676 +3 ========================================== Hits 355 355 - Misses 271 274 +3 Partials 47 47 ```

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[thaJeztah]

Doh! my commit description says errors.Is() but using errors.As(). Probably a nit, but let me fix that to prevent confusion.

[thaJeztah]

Fixed 😅