Cleaning up Constraints after failed rule

[andrewlee94]

From a discussion in IDAES: https://github.com/IDAES/idaes/pull/177

When a constructing a Constraint using a rule, the Constraint component is first created and then populated by the rule. If the rule fails for some reason but the Exception is handled by something, the Constraint object remains attached to the model.

It would be good if Pyomo could clean up when things go badly wrong in this way.

[mrmundt]

@andrewlee94 - Just checking, is this still an issue? We are going through old bug reports to check if they are still present in the current version of Pyomo.