Quick fixes for REST Read endpoints

I've been able to test the changes you've made in https://github.com/DSpace/DSpace/pull/666, and had to make some fixes to get the endpoints to work. I've tested read/get of community, collection, item, bitstream, and they were Error 500. Some of the latest changes, to add finally context.abort everywhere likely caused this.

I've also changed ```/rest/bitstreams/:id/retrieve``` from ```@POST``` to ```@GET```. It just seems normal, at least how I understand the API. i.e. ```<img src="https://dspace.example.com/rest/bitstreams/:id/retrieve"/>```.

Also, my testing app for the read-only version of DSpace REST API is the following. It's helpful for me to run through and see all the data coming from the REST API properly. I'm going to add login, Post, Put, Delete operations to this, so I can test all of the operations. In the process this testing app turns into a light-weight UI for DSpace.
https://github.com/peterdietz/dspace-rest-play

If you agree with these commits of mine, and Accept/Merge, I think that also updates your Pull Request to DSpace/DSpace.

By the way, thank you very much for this contribution.