HACKING.rst: Add note about asking for dev permissions

See merge request !587

A missing language specifier on a literal include directive results in yaml
being rendered as python code. This commit adds the :language: tag to that
directive.

junction-elements.rst: Add missing language specifier to literalinclude

See merge request !591

Normally we'd only need it in the case of scheduling a weakly cached build,
but to allow caching of failed builds we need to be able to distinguish
between cached successes and cached failures
for both strong and weak cache keys.

To allow other cache lookup codepaths to look up via the weak key
requires changes through the call stack to consult which key to use,
and cache invalidation of the saved state when it changes.

This just puts the metadata in place,
we're adding code paths to add failed builds later.

When we later add cached failures it needs to not treat them as successes.

This creates an artifact when element assembly fails too,
and if it's the right kind of exception uses the now-included install directory
similarly to if it had returned successfully.

If there's a failure during install the artifact contains any installed files,
but may contain nothing at all.

This flags up a failure and if run in an interactive prompt
permits the user to attempt a rebuild.

This allows the scheduler to move jobs from the current queue to the next.

As a result of this change later queues than the build queue
mustn't skip a cached failure, so the logic is specialised to build queues only.

Closes #76.

Store failed builds in the cache

Closes #76

See merge request !475

Trying to checkout an element that has no cached artifacts should be and
handled failure. See #447.