* Make the error variable always named err.
Sometimes the error variable was named 'err' sometimes 'e'. Sometimes
'e' refered to an EDNS or string and not an error type.
* Use t.Errorf instead of t.Logf & t.Fail.
This will allow RRSIG.Sign to use generic crypto.Signer implementations.
This is a interface breaking change, even if the required changes are most
likely just type asserions from crypto.PrivateKey to the underlying type or
crypto.Signer.