Fix #51: Handle subprocess.CalledProcessError

CPT will not pack binaries into packages which fail during build. Henceforth we should be skipping tests if we are to package binaries into bundles.

For example, we will need this in Windows to build NSIS installers:
```python2 cpt.py --current-dev=nsis --no-test```
or 
```python3 cpt.py --current-dev=nsis --no-test```

This patch is an extension of the ```check_call``` and ```check_output``` implemented by @nneonneo