st2-check-pylint-pack doesn't check if requirements installed successfully

[LindsayHill]

When make lint runs, it creates a virtual environment and installs any pip packages listed in requirements.txt.

If any of those packages fail to install, e.g. because additional system packages like libmlx2-dev are required, the script carries on, and runs pylint. I think it should exit if any packs fail to install.

Example output:

Command "/root/StackStorm/st2contrib/virtualenv/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-GaoJ7u/lxml/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-xA8Jhj-record/install-record.txt --single-version-externally-managed --compile --install-headers /root/StackStorm/st2contrib/virtualenv/include/site/python2.7/lxml" failed with error code 1 in /tmp/pip-build-GaoJ7u/lxml/
--> No pylint issues found in actions.
--> No pylint issues found in sensors.