added warning message if hysds_io returns 404 not found

[DustinKLo]

in io-met.py when attempting to add users to the allowed_users field it will print a 404 client error if the hysds_io doesn't exist in ES

404 Client Error: NOT FOUND for url: http://100.##.###.###:8878/api/v0.1/hysds_io/type

this change will instead change it to a WARNING

[DustinKLo]

testing a build with a brand new job that doesn't exist in my cluster

[CI] Build for: hysds-io-hello_world_notebook:main and file hello_world_notebook
Running IO-Met on: container-hysds_hello_world_notebook:main docker/hysds-io.json.hello_world_notebook main hysds-io-hello_world_notebook:main
WARNING: hysds_io not found: hysds-io-hello_world_notebook:main, cannot merge allowed_accounts

job found in ElasticSearch

$ curl "localhost:9200/hysds_ios-grq/_doc/hysds-io-hello_world_notebook:main?pretty&_source=false"
{
  "_index" : "hysds_ios-grq",
  "_type" : "_doc",
  "_id" : "hysds-io-hello_world_notebook:main",
  "_version" : 1,
  "_seq_no" : 23,
  "_primary_term" : 15,
  "found" : true
}