Pad onion errors

[TheBlueMatt]

The spec recommends a fixed length to avoid giving away any fingerprinting on the error type. I believe we do not currently do this, but we should.

[freddiekrugerrand]

The spec requires pad_len = 256 - failure_len (where failure_len = 2 bytes failure_type + len(failure_data)). IIUC this is already done here.

[TheBlueMatt]

Oh, damn, good catch. Well, the spec may change the target pad so we'll want to do that when the BOLTs change.