text: Title: imagePullSecrets should log warning if secret does not exist
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  #### What happened:
  
  imagePullSecrets can added to a pod manifest to reference a secret containing registry credentials. Currently, if you add the name of a secret that does not exist, there is no warning or error. 
  
  #### What you expected to happen:
  
  I would expect there to be an error and a failed deployment if the secret does not exist/cannot be found in the namespace. 
  If people are pulling public images, they may still wish to authenticate to a registry, for example dockerhub, before pulling the image, in order to utilise their subscription account rate limits. Currently if they use imagePullSecrets and the referenced secret does not exist, they may assume they are authenticating to their account when they are not, and will still be able to pull public images without any warning that authentication is not taking place. 
  
  #### How to reproduce it (as minimally and precisely as possible):
  
  Add `imagePullSecrets: myfakesecret` to a pod spec, and pull images from docker.io.
  