Provide description for authentication error in case for POST of RO-user

[denyago]

Would be nice to have a description for auth error, if a read-only user tries to post some data.

Use case: As a RO-user, I want to do a request like this: POST http://127.0.0.1:7474/db/data/node/2/traverse/node. I want to get a description, that I've been authenticated, but can't 'write' data.

Maybe, this would be other HTTP code. 403 for example.

Thanks!

[jexp]

Feel free to send an PR :)