Raise suitable exception when stack validation fails

fixed Bug#1191009

If nova returns an error looking up images during stack validation, we
return a StackValidationFailed, but we should probably catch
this and return ServerError instead, since this is really an internal
error not a validation failure.

Change-Id: Id509b2fa9f78e197cacfe44bae6c3db37cfd6179
Merge "Raise suitable exception when stack validation fails"