Improve package download error while offline

Now we see something like:

  $ curmeteor
  Unable to refresh catalog (are you offline?)

  Could not download package iron:dynamic-template@0.4.1: getaddrinfo ENOTFOUND
  Could not download package iron:router@0.9.4: getaddrinfo ENOTFOUND
  Could not download package iron:layout@0.4.1: getaddrinfo ENOTFOUND
  Could not download package iron:core@0.3.4: getaddrinfo ENOTFOUND
  Could not install all the requested packages.