Entity not found cause error 500

[theofidry]

When submitting a POST request to create a new form with an inexistent entity in the body, this cause a 500 error instead of a 400 one.

Originally reported by @aledeg in api-platform/api-platform#88.

[teohhanhui]

So this is due to the use of EntityManagerInterface::getReference with an invalid (non-existent) id...

[teohhanhui]

We need to catch the Doctrine\ORM\EntityNotFoundException

[theofidry]

Yep, sorry I don't have the time to really look in the code and provide a fix right now :/

[soyuka]

I could work on a fix asap, just need some review on #552 first.