Explicitly check that project.conf contains a name. This resolves
the issue of the provenance check from _yaml.py incorrectly reporting
the offending file as the default_config_node projectconfig.yaml
when no name key exists in the pre_config_node dict.

buildstream/_project.py: Report if project.conf is missing name

See merge request !680

SkipJob can now be raised within the context of a timed_activity() to
trigger a SKIPPED message instead of a FAILURE message.

This changes the previous usage of SKIPPED within messages to use the
INFO MessageType (used within cascache.py).

Element._pull() now performs its actions from within a timed_activity()
to match Element._push() and allow for SkipJob to be raised.

"Downloaded artifact <cache key>" has been reworded to "Pulled artifact
<cache key>" for consistency with Element._push().