An exception thrown during GraphModel.OnEnable when creating/loading a Graph can cause a memory leak

Steps to reproduce:

1. Define a graph that modifies itself during OnEnable (eg : creates a node)
2. Instantiate an intance of that graph in code (using GraphDatabase methods)
3. Count the number of objects of your graph type

Actual results: 

Count will be 1

Expected results: 

Count will be 0

Reproducible with versions: 6000.6.0a1, 6005.0a6

Not reproducible with versions: 6000.4