Update our instructions/examples regarding how we declare remote caches

See merge request !932

There are arguments for and against using onerror - see issue #735
for details.

Use _force_rmtree instead of custom code.

Closes #735

See merge request !927

Valentin David authored Nov 06, 2018 and richardmaw-codethink committed Nov 07, 2018

Root directory was marked as a non-artifact mount, so not using
SafeHardLink.  However integration commands executed with write access
to the root directory.

Fixes #749