Handle 404 in maasEnviron.StopInstances
Retry release individually in certain cases
Improved error handling and test
Test recursive case
Log warnings for release failure
Logging improvement
Merge branch 'master' into maas-404
Merge pull request #1127 from voidspace/maas-404

Handle missing nodes and permission errors when releasing MaaS nodes

When releasing MaaS nodes a 400, 403 or 404 response means that one or more nodes had errors and none have been released. The error is that the node is no longer available, so we can ignore the error (it's already been released) but we still need to release the other nodes. 

Fixes bug 1319016.