EdgeAgent: Fix error handling in GetTwin

[varunpuranik]

The issue -

• Currently the EdgeAgentConnection gets the EdgeAgent config from the twin once initially, and thereafter via a desired properties update callback or periodic GetTwin call.
• The twin that is returned in the Reconcile loop is the cached value.
• If getting the twin and parsing it fails, an error is returned to the Reconcile loop (via the IConfigSource interface) instead.
• Because of this, if the periodic GetTwin call fails, an error gets cached and returned for each of the subsequent Reconcile loops.

The fix

• The GetTwin code has retry to deal with transient errors. This PR updates that code to get a new ModuleClient every time. This takes care of the case where the SDK throws an ObjectDisposedException on the GetTwin call and goes into a bad state. The subsequent GetTwin calls should now succeed.
• If the GetTwin fails, it is not regarded as bad config and not returned to Reconcile. Instead the previous cached config is returned. Only if a GetTwin succeeds but parsing the twin fails, it is regarded as bad input and returned to the Reconcile logic as an Exception.

[myagley]

catch if not fatal?