Worker: better error messages on credentials

[josephjclark]

When the worker fails to load a credential, it'll just log a generic error message.

We need to ensure that if Lightning returns an error on the credential, that the error makes it back to the run logs.

As part of this issue, I also want to make sure we've got good unit and integration tests around this area (at a quick look it seems not too bad)

See OpenFn/lightning#1842

[josephjclark]

One fun thing here is that this happens before the run technically starts. It's part of the lost logs without a step id. I think lightning will just ignore it.

Anyway, so long as we throw a better error message out of the credential loader, we'll show the message as the exit reason and everything will be fine.