This commit updates the encoding/decoding for the onion errors to
properly match BOLT-0004. This commit does to major things: we increase
the max message size to 256 bytes to account for recent modifications
to the ChannelUpdate message, and also fold in the error code into the
failure message encapsulation itself.
In this commit onion routing error from BOLT#4 have been added, the
initial error construction have been splitted on two parts: The first
part which contains the actual data will reside inside the lnwire
package because it contains all necessary function to decode/encode
the data. The second part obfuscation and hmac checking will resides
inside the lighting-onion package because it requires the key
generation.
