Validate process graph when creating batch job (instead of when starting it)

[m-mohr]

I'm currently experiencing an issue with a process graph as given in Open-EO/openeo-python-driver#146. If I submit it via POST /jobs, then it is accepted. But when I queue it, I get a validation error message that says that FeatureCollection is not supported.
I'd expect that the back-end already complains about high-level issues when I submit the process, not only when I queue it.