Backport unhandled exception fix in cascache.py

See merge request !602

This is needed to build docs without installing BuildStream.

  o Added feature to allow running shell commands

  o Removed unused codepaths to launch bst commands through the script,
    this program is anyway only used to parse and run the session files.

  o Fixed previous patch to run BuildStream uninstalled with
    the subprocess module, as running it in the same interpretor
    causes a bunch of problems.

Build documentation without installing (backport to 1.2)

See merge request !606

Qinusty authored Jul 23, 2018 and Tristan Van Berkom committed Aug 06, 2018

This address issue #491.

When attempting to run buildstream with a configuration specifying
a cache quota larger than your available disk space, buildstream
will alert the user and exit.

Note:
This takes into consideration your current cache usage and
therefore restricts the overall size of your artifact cache folder.