orderly_pull_dependencies(): relax requirement for a complete orderly.yml

[jeffeaton]

Currently, orderly_pull_dependencies() requires a complete orderly.yml including script file and artefact to run.

For example, if no artefact is listed yet:

> orderly::orderly_pull_dependencies()
Error in check[[x]](raw[[x]], config, filename) : 
  At least one artefact required

For development workflow, if valid dependencies exist in the orderly.yml, it would be nice for orderly_pull_dependencies() to proceed to fetch them and raise a warning about the missing components of the orderly.yml, similar to the behaviour for orderly_develop_start().

Thanks,
Jeff