Handle checkout failure for unbuilt elements

Description

Trying to checkout an element that has not been build yet (that has no artefact cached yet) leads to a BUG + stack trace.

Changes proposed in this merge request:

• Raise an ElementError in Element.stage_artifact() if the element has not yet produced an artifact, as suggested by the doctring.
• Advise on running bst build first as details of the raise exception.
• Ensure that the exception is caught and that the advice reached the caller.

This merge request, when approved, will close issue/bug: #447 (closed)