simplexml error handling php

Estou usando o seguinte código:

function GetTwitterAvatar($username){
$xml = simplexml_load_file("http://twitter.com/users/".$username.".xml");
$imgurl = $xml->profile_image_url;
return $imgurl;
}

function GetTwitterAPILimit($username, $password){
$xml = simplexml_load_file("http://$username:$password@twitter.com/account/rate_limit_status.xml");
$left = $xml->{"remaining-hits"};
$total = $xml->{"hourly-limit"};
return $left."/".$total;
}

e obtendo esses erros quando o fluxo não puder se conectar:

Warning: simplexml_load_file(http://twitter.com/users/****.xml) [function.simplexml-load-file]: failed to open stream: HTTP request failed! HTTP/1.0 503 Service Unavailable

Warning: simplexml_load_file() [function.simplexml-load-file]: I/O warning : failed to load external entity "http://twitter.com/users/****.xml" 

Warning: simplexml_load_file(http://...@twitter.com/account/rate_limit_status.xml) [function.simplexml-load-file]: failed to open stream: HTTP request failed! HTTP/1.0 503 Service Unavailable

Warning: simplexml_load_file() [function.simplexml-load-file]: I/O warning : failed to load external entity "http://***:***@twitter.com/account/rate_limit_status.xml"

Como posso lidar com esses erros para que eu possa exibir uma mensagem amigável ao invés do que é mostrado acima?