Produce more meaningful exception messages in nested stacks

This produces a nested exception like:
 'ValueError: resources.nested.resources.my_server: it is broken, sorry'

This re-uses the path mechanism that StackValidationFailed exception
uses.

Change-Id: Id5204c15ee96784e04522ab3c5a8e66900f9a1d3
Closes-bug: 1459837
2 files changed
tree: 193cdee781081a5ea3036583638323c642a2b2e6
  1. common/
  2. functional/
  3. scenario/