add error handling
fix problem
[protocolbuffers\message] string type should keep an object which
implements __toString method.

before this commit. ProtocolBuffers\Message implicitly convert object to
string when the field type is string.
that behavior is very simple. but sometimes want to keep object which
implements __toString.

class MyString
{
    protected $buffer = "";
    public function append($string){$this->buffer .= $string}
    public function __toString(){return $this->buffer;}
}

$mystring = new MyString();
$person = new Person();
$perseon->setDescription($mystring);

// this is easy to modify string!
$mystring->append("hello!");

// call __toString method internally when converting object to protocol
// buffers message
echo $person->serializeToString();