You could do any of the following:
Set a global error handler (that will handle WARNINGs as well), for all of your unhandled exceptions: http://php.net/manual/en/function.set-error-handler.php
Or by checking the return value of the file_get_contents function (with the === operator, as it will return boolean false on failure), and then manage the error message accordingly, and disable the error reporting on the function by prepending a "@" like so:
$json = @file_get_contents("file", true);
if($json === false) {
// error handling
} else {
// do something with $json
}