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

[dwstern]

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.