fix: reslice error log to remove nil values

[JamieEdge]

Fixes #80.

Based on #78 to update tests and avoid causing a failure due to a nil value being expected, as referenced below.

retry-go/retry_test.go

Line 167 in 54c42e0

expectedErr := Error{testErr, nil}

⚠️ This may be considered a breaking change because some consumers of the package may expect the length of the error to be the maximum number of attempts rather than the actual number.

[JaSei]

thank you for your PR