Prevent ConfigPusher from killing its thread

The original code promoted pretty much every error to an unhandled
RuntimeException. Use the exceptions already in place to handle erros
occuring. Also handle IO error as a retriable operation.

Change-Id: I666d66e89993f0f9d18afb6ec680e64a50cefec1
Signed-off-by: Robert Varga <rovarga@cisco.com>