Deleted Stack Fix and ValidationError for DescribeStacks.

  * stop backed from trying to iterate over empty list of deleted stacks.
  * Update DescribeStacks to throw ValidationError if stack name or id doesn't exist.
Add test for bad describe stack request
create_stack returns stack_id
test delete stack
python 2.6 assert_raises
remove some test details for 2.6
Merge pull request #240 from joekiller/fix/describe_stacks_add_validation_error

Fix/describe stacks add validation error