Improve conductor error cases when unshelving

This commit handles different cases when the key 'shelved_image_id'
is not defined or None. Also it tries to improve the error messages.

Closes-Bug: #1307416
Change-Id: Ia2d0a183eefccdfd6f612d86f60cd2930c1a4b23
Merge "Improve conductor error cases when unshelving"