title: Verify if an iam username already exist
body: <p>Do you know how can I verify if an iam username already exist?</p>
  
  <p>Im trying with code below, using get_user, but when user dont exist I got an error: <code>The user with name teste cannot be found</code>.</p>
  
  <p>Do you know the way to do this?</p>
  
  <pre><code>c = boto.iam.connect_to_region("us-east-1")
  while True:
      username = raw_input("Username:")
      password = raw_input("Password:")
      if not username or not password:
          print "Please enter a valid username and password"
      if c.get_user(username):
          print "Username already exist, please enter a different username..."
  </code></pre>
  
  <p>New try:</p>
  
  <pre><code>while True:
      username = raw_input("Username:")
      password = raw_input("Password:")
      if not username or not password:
          print "Please enter a valid username and password"
      try:
          if not c.get_user(username):
              print "Username valid"
      except BotoServerError, e:
          print "Username already exist
  </code></pre>