Remove assertion from `obix_batch_process`

Changes in this commit:
* Fixes #44 
* Adds error handling on obix:Batch input which is a whole lot more graceful than a segfault.

```bash
$ ota  |curl -XPOST --data @- -H Expect: http://localhost:4242/obix/batch
oBIX Template Adaptor
Tyler Watson <tyler.watson@nextdc.com>

<?xml version="1.0"?>
<err is="obix:ServerErr" name="General error contract" href="/obix/batch" displayName="obix:Batch" display="Provided obix:BatchIn document is invalid"/>
```