Be more robust against errors whilst aborting (crash due to recursion)

Right now we don't handle conditions like out of file descriptors safely, but instead PANIC because we fail recursively while aborting the transaction:

```
2017-08-07 00:40:03.766 UTC [17342][4/67561] : [2-1] [app=[unknown]] LOG: connection authorized: user=citus database=citus SSL enabled (protocol=TLSv1.2, cipher=ECDHE-RSA-AES256-GCM-SHA384, compression=off)
2017-08-07 00:40:03.850 UTC [17342][4/67562] : [3-1] [app=psql] ERROR: epoll_create1 failed: Too many open files
2017-08-07 00:40:03.850 UTC [17342][4/67562] : [4-1] [app=psql] STATEMENT: INSERT INTO anonymized SELECT * FROM anonymized
2017-08-07 00:40:03.851 UTC [17342][4/0] : [5-1] [app=psql] ERROR: epoll_create1 failed: Too many open files
2017-08-07 00:40:03.851 UTC [17342][4/0] : [6-1] [app=psql] WARNING: AbortTransaction while in ABORT state
2017-08-07 00:40:03.851 UTC [17342][4/0] : [7-1] [app=psql] ERROR: epoll_create1 failed: Too many open files
2017-08-07 00:40:03.851 UTC [17342][4/0] : [8-1] [app=psql] WARNING: AbortTransaction while in ABORT state
2017-08-07 00:40:03.851 UTC [17342][4/0] : [9-1] [app=psql] ERROR: epoll_create1 failed: Too many open files
2017-08-07 00:40:03.851 UTC [17342][4/0] : [10-1] [app=psql] WARNING: AbortTransaction while in ABORT state
2017-08-07 00:40:03.851 UTC [17342][4/0] : [11-1] [app=psql] ERROR: epoll_create1 failed: Too many open files
2017-08-07 00:40:03.851 UTC [17342][4/0] : [12-1] [app=psql] PANIC: ERRORDATA_STACK_SIZE exceeded
```