Flows should be aborted on policy errors

**Is your feature request related to a problem? Please describe.**
When creating a flow that uses an expression policy, an error in the policy causes the last step of the flow to be executed. This can be a potential security issue.

If a policy throws an error, the intended behavior of the flow is undefined. The only safe default is to abort the flow. In my experiments, I had for example users being created although the policy should have prevented it, due to a policy error.

**Describe the solution you'd like**
The default for any created flow should be to abort if there is a policy error.