fix(#794): Wrapping codes.OK should not cause panic

[floppyzedolfin]

An panic was raised in wrappedErrField.Error() when PingError was used without specifying the ErrorCodeReturned.
See #794 for a description of the original issue, with context.
It turns out it is possible to build a wrappedErrFields which has a wrappedErr == nil.

Changes

Added some safety nets in the calls accessing the err.wrappedErr field.

Verification

Added a unit test that covers the "usual" scenario (when the ErrorCodeReturned was properly set) and the edge-case scenario (when no ErrorCodeReturned was set).

[johanbrandhorst]

Thanks!