2.7.0-beta: xmrig is not exit when wrong configuration
Example: work is impossible (bad threads configuration), no GPU is configured.

```shell
./xmrig-amd -c nova_amd17 
 * VERSIONS     XMRig/2.7.0-beta libuv/1.9.0 OpenCL/2.0 gcc/4.9.2
 * CPU                Intel(R) Celeron(R) CPU  J1800  @ 2.41GHz x64 -AES
 * ALGO         cryptonight, donate=5%
 * POOL #1      monero.napaster.name:14444 variant 1
 * COMMANDS     hashrate, pause, resume
[2018-06-17 15:01:31] compiling code and initializing GPUs. This will take a while...
[2018-06-17 15:01:31] #0, GPU #3 , intensity: 257 (8/256), cu: 24
[2018-06-17 15:01:31] Error CL_INVALID_BUFFER_SIZE when calling clCreateBuffer to create hash scratchpads buffer.
[2018-06-17 15:01:31] use pool monero.napaster.name:14444 51.15.55.162
[2018-06-17 15:01:31] new job from monero.napaster.name:14444 diff 120001 algo cn/1
[2018-06-17 15:01:39] new job from monero.napaster.name:14444 diff 120001 algo cn/1
[2018-06-17 15:01:48] new job from monero.napaster.name:14444 diff 120001 algo cn/1
[2018-06-17 15:02:18] Ctrl+C received, exiting
[2018-06-17 15:02:18] no active pools, stop mining
```