Add details to "non-empty queue on spawn exit"
Add details to "non-empty queue on spawn exit"

Improve the error reported when the child process exits before the
whole queue is processed:
 - use assert.AssertionError instead of plain Error
 - fill actual and expected properties
 - include the description of items remaining in the queue

ryanbmilbourne/spectcl,ViaSat/spectcl,stacywile/spectcl,ryanbmilbourne/spectcl,stacywile/spectcl