All of the errors which subclass from BstError have their first
positional argument as message, LoadError should follow this
ordering for consistency.

This replaces the helper method by adding a 'get_provenance' on the node
directly

- Adapt all call sites
- Delay getting provenance wherever possible without major refactor

- Adding 'get_sequence' on MappingNode to access sequences in a mapping
- Adding 'sequence_at' on SequenceNode to access sequences in a sequence
- Adding 'mapping_at' on SequenceNode to access mappings in a sequence

Using "*_at" in sequences allows us to quickly understand if we are
dealing with a sequence or a mapping.

This allows to get a mapping node from another 'MappingNode',
replacing 'node_get(my_mapping, key, type=dict)'

Also changes all places where 'node_get' was called like that by
the new API.

Instead of having methods in Context forward calls on to the Messenger,
have folks call the Messenger directly. Remove the forwarding methods in
Context.

project_overrides and project_extra_specs were moved in 24c0de16.
However, their removal was accidentally reverted in f4dbce44.
This patch removes the unnecessary code.

Make a copy of the global list rather than using the global
list.

This was originally addressed by:
e1420437a2d82af099bdb3d273823effbea35ce4
and then again by 1ee4a4ba3a91e27515967d5e2daaedf664f10c78.

This creates a new ArtifactRemote class, derived from CASRemote that
extends initialisation to check for an artifact service. This drops the
remote early rather than raising an error on method not found each time
it tries to use it.

Fixes #1025

This was discussed in #1008.

Fixes #1009.