auto-validation of graphs

[jdries]

The validation endpoints allows a backend to convey errors but also warnings to the user. In the python client, we now do this automatically before submitting the graph, because we noticed that users never did it by themselves, loosing the opportunity to get important information up front.
The web editor could do the same thing.

[m-mohr]

I'm not really sure at which point I should execute this, maybe it's better to draw the users attention more towards this functionality? It is not very easy to find...