* chore/backend: move dashboard errors to dashboard service
Dashboard-related models are slowly moving out of the models package and into dashboard services. This commit moves dashboard-related errors; the rest will come in later commits.
There are no logical code changes, this is only a structural (package) move.
* lint lint lint
Moves/refactor Grafana specific functionality related to plugin dashboards
out to specific services for importing dashboards and keep app plugin dashboards
up-to-date.
Fixes#44257
