Editor crashes on file open when failing to load included workflow

[glopesdev]

To reproduce: open a workflow file containing missing included workflows, or invalid paths to embedded assemblies which cannot load (i.e. an included workflow pointing to a missing package).

This is a regression caused by fixing #800 with eager loading without accounting for potential exceptions raised during load. If this is the case, it might be okay to simply catch any exceptions to ensure ArgumentRange never throws, and wait for the error to be thrown during proper workflow building.

[PathogenDavid]

I noticed while reviewing #1890 that with this fix it's possible that an exception in the wrong place (namely, after workflow is initially assigned) could leave IncludeWorkflowBuilder in an inconsistent state and the exception would not show up later as intended.

The easiest fix is probably to store the newly-created workflow in a local and only assign it to this.workflow at the very end of the if (workflow == null || lastWriteTime > writeTime) block in EnsureWorkflow.