Improve experience when submitting a Challenge Response

[ebekker]

As documented in #49, if a Challenge has been unsuccessfully submitted and transitions into the invalid state, then that Challenge can no longer be submitted and new Identifier Authorization request should be issued.

To help the user experience with this situation, we should update the relevant cmdlets to test for the current (or last status) received for a given Challenge, and if it's not in Pending state, then print out a warning and abort. The -Force flag will allow a user to force the retry on an existing Challenge in either case.