Improve error handling about cozy-data-system

[jsilvestre]

I think this should be handled here https://github.com/cozy/cozy-data-system/blob/master/server/middlewares/errors.coffee

[aenario]

We can't.

At this point, the error's message is already [Object object]. We want an Error object ASAP (so we have a stack trace) and then just pass it around with no changes.

Our code should aways be return callback err
or return callback new Error('our own message')
it should never be callback new Error(err)

We started doing this because cradle would often send a string or an object {} as the err. A PR has just been merged in cradle flatiron/cradle#235, so it will now always pass Error object as reply. Once this is published in npm, we should update it and remove all new Error(err) from the DS.

[jsilvestre]

Fair enough :)

[frankrousseau]

:+1