title: Warning message when using closness() in igraph
body: <p>I am looking to calcuate the closeness centrality measure of a  graph of USairports for a set of vertices: ATL, JFK, ORD, LAS</p>
  
  <p>The code I have is as follows: </p>
  
  <pre><code>closeness(USairports,vids=c("ATL","JFK","ORD","LAS"),mode="all")
  </code></pre>
  
  <p>This gives me the warning message</p>
  
  <blockquote>
    <p>"In closeness(USairports, vids = c("ATL", "JFK", "ORD", "LAS"), mode =
    "all") :   At centrality.c:2784 :closeness centrality is not
    well-defined for disconnected graphs"</p>
  </blockquote>
  
  <p>What does this warning essentially mean?</p>