Check for existence of trait and throw a more meaningful exception than 'Undefined index'

Issue #234 shows, that on the first glance the error message *"exception 'Nette\FatalErrorException' with message 'Undefined index: FlorianWolters\Component\Core\CloneNotSupportedTrait' in C:\dev\apigen\ApiGen\ReflectionClass.php:873"* is confusing for the user. It was the same for me.

So I want to improve this part with this pull request, so that the user gets an idea how to solve the issue.