Fix flakiness in fabric_bench

`{error, timeout}` sometimes fails, especially on FreeBSD workers.

So handle errors in the docs callback and also half the batch size.

Example of error:
```
module 'fabric_bench_test'
 fabric_bench_test:99: -with/1-fun-1- (t_default_doc_size)...*failed*
   in function fabric_bench:all_docs_cb/2 (src/fabric_bench.erl, line 179)
     called as all_docs_cb({error,timeout},7)
```
2 files changed