Problem: pip install return code is not returned

Solution: pass it properly to buildout via a call to sys.exit

news/586.bugfix

+Return code of pip install subprocess is now properly returned to buildout.

src/zc/buildout/pip_support.py

-try: 
+try:
     # support pip>=19.0
     from pip._internal.cli.main import main
-    pip_install_cmd = 'from pip._internal.cli.main import main; main()'
+    pip_install_cmd = (
+        'from pip._internal.cli.main import main; sys.exit(main())'
+    )
 except ImportError:
     # support pip>=10.0
     from pip._internal import main
-    pip_install_cmd = 'from pip._internal import main; main()'
+    pip_install_cmd = 'from pip._internal import main; sys.exit(main())'