Non existant namespace alias throws a PHP Notice

Proposed tests and fix

http://github.com/merk/common/commit/65e8c10b74201c39dbe291c36230205d9b08ed65

In commit: http://github.com/doctrine/common/commit/ad49a676269af368563bd9a848c904b81a825622

I fixed this issue.
The validation on unexistent classes cannot be done correctly, otherwise it'd conflict with phpDoc tags.

What will your change do if there are multiple implementations of the Reader working on entities that dont have common namespaces, but still use namespaces?

Would it not make more sense to skip something that the Reader doesnt understand, rather than throwing an error?

(For example, Symfony2's validator system might have namespaced validators, which would then cause the 'Doctrine' reader to throw an exception)

Yes, you're right...

I think we'll have to silently bypass unknown aliases... this makes debug harder, but we don't have any other option. =(

Reopening it