Handle 0x80 completion code for DCMI capabilities

[gebn]

The BMC is entitled to omit everything after the completion code (apart from the checksum), so the error should be able the non-normal completion code, not about the missing defining body code.

[gebn]

If a non-zero completion code is returned, do we want to even attempt to decode lower layers? Probably not.

[gebn]

Also handle RAKP completion codes - these are different (non-overlapping) from message codes. E.g. 0x01 is "insufficient resources" when creating a session, but 0xc0 is used for "node busy" in response to a command.

[gebn]

This is now mostly fixed; the only one left is session establishment requests (non-commands). We don't currently retry these if we get an insufficient resources error. This is not the end of the world.