Add error handling to SSM WaitUntilCommandExecuted

[Will Moore]

What I'd like:
Currently the SSM WaitUntilCommandExecuted is not reporting encountered errors during execution time. These errors need to be caught and made available to the user so problems can be tracked and investigated for remediation. Since WaitGroups are not a priority target for GA this will be done sequentially until such a time that WaitGroups are implemented.

Any alternatives you've considered:
Add error handling here

[Will Moore]

addressed in pull #65