Validate caslib exists in /createpublishdest.py cas before creating publishing destination

Attempting to create CAS pub dest in nonexistant caslib with createpublishdest.py half-works but should fail.

You can run createpublishdest.py using a command such as the following, where the argument -c nonexistantcaslib passes in the name of a CAS library which does not exist:

./createpublishdest.py cas -n newcasdest -s cas-shared-default -c nonexistantcaslib -t thetable

The command succeeds, reporting that it has created a publishing destination. However, the publishing destination does not appear in the SAS Environment Manager Publishing Destinations page. Attempting to re-run the same command results in an HTTP 409 response something like this:

{"errorCode":21904,"message":"The destination name \"newcasdest\" is already in use. You must specify a unique name.","details":["traceId: 66a865672d30378b","path: /modelPublish/destinations/"],"links":[],"version":2,"httpStatusCode":409}

Modify createpublishdest.py so that for a CAS publishing destination, it validates that the CAS library exists before proceeding to attempt to create the publishing destination which uses it.

Start in createpublishdest.py and trace the cas publishing-destination path invoked by the example command. Add validation that the supplied CAS library exists before attempting destination creation. Run the command with nonexistantcaslib and verify it fails without reporting success or leaving a destination that causes an HTTP 409 on retry.