Better error reporting on AWS failures

[Kenneth Koski]

As a follow-up to https://bugs.launchpad.net/charm-aws-integrator/+bug/1821785, it would be nice if the aws-integrator charm could report errors a little better. Right now if there's an error like hitting the subnet tag limit, juju status just shows "error while granting requests; check credentials and debug-log", and CI processes won't necessarily be dumping the debug log anywhere handy, so the deploy just times out and there's no great introspection. If `juju status` were instead able to show something like the exception from debug-log, that would be much more helpful:

    charms.layer.aws.AWSError: An error occurred (TagLimitExceeded) when calling the CreateTags <...> too many Tags