Use more specific error codes

[joaquimrocha]

Recently we've had an issue when OSTree cannot find a file object. We implemented a workaround based on the detection of the G_IO_ERROR_NOT_FOUND but this error is being returned in other places, for different reasons so it's easy that eventually we end up matching the same error when the cause is not a missing file object.

I would like to know whether this could be a real a problem and, if so, propose to return more specific errors for (at least) the places where the G_IO_ERROR_NOT_FOUND is being returned.

[cgwalters]

Hmm. I think this could be a symptom of #813 - if another operation is happening concurrently, it absolutely could delete pending objects.