Alexander Neumann a9af896ddd Add verbose error when marshalling a node fails
This code is introduced to watch for issue #529, in which two users
describe that restic failed encoding a time in a node to JSON with the
error message:

    panic: json: error calling MarshalJSON for type *restic.Node: json: error calling MarshalJSON for type time.Time: Time.MarshalJSON: year outside of range [0,9999]

The error message now is:

    panic: Marshal: json: error calling MarshalJSON for type *restic.Node: node /home/fd0/shared/work/restic/restic/.git/hooks/applypatch-msg.sample has invalid ModTime year -1: -0001-01-02 03:04:05.000000006 +0053 LMT
2016-09-17 10:43:04 +02:00
..
2016-09-15 22:29:49 +02:00