IR stack dump for a Consul service with no endpoints

[ewbankkit]

Describe the bug
I have configured a service with an Ambassador Mapping that points to a ConsulResolver.
For some reason (I'm still investigating) in the IR phase I get the error:

2019-05-10 13:31:18 diagd 0.61.0 [P53TAmbassadorEventWatcher] ERROR: cluster_demo_app_er_round_robin: <RichStatus BAD error=no endpoints found, disabling cluster hostname=ambassador-poc-6dfb5959df-gt98k version=0.61.0>

which comes from this code: https://github.com/datawire/ambassador/blob/e693d96a0df01e9e9dce79e98b6cf0fcfd5ae02e/ambassador/ambassador/ir/ircluster.py#L280-L296

but then there is a stack dump:

2019-05-10 13:31:18 diagd 0.61.0 [P53TAmbassadorEventWatcher] ERROR: could not reconfigure: 'targets'
2019-05-10 13:31:18 diagd 0.61.0 [P53TAmbassadorEventWatcher] ERROR: 'targets'
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/ambassador-0.0.0.dev0-py3.6.egg/ambassador_diag/diagd.py", line 590, in run
    self.load_config_watt(rqueue, url)
  File "/usr/lib/python3.6/site-packages/ambassador-0.0.0.dev0-py3.6.egg/ambassador_diag/diagd.py", line 708, in load_config_watt
    self._load_ir(rqueue, aconf, fetcher, scc, snapshot)
  File "/usr/lib/python3.6/site-packages/ambassador-0.0.0.dev0-py3.6.egg/ambassador_diag/diagd.py", line 797, in _load_ir
    self.check_scout("update")
  File "/usr/lib/python3.6/site-packages/ambassador-0.0.0.dev0-py3.6.egg/ambassador_diag/diagd.py", line 819, in check_scout
    feat = ir.features()
  File "/usr/lib/python3.6/site-packages/ambassador-0.0.0.dev0-py3.6.egg/ambassador/ir/ir.py", line 583, in features
    cluster_endpoints = cluster.urls if (lb_type == 'kube') else cluster.targets
  File "/usr/lib/python3.6/site-packages/ambassador-0.0.0.dev0-py3.6.egg/ambassador/resource.py", line 94, in __getattr__
    return self[key]
KeyError: 'targets'

which comes from:

https://github.com/datawire/ambassador/blob/e693d96a0df01e9e9dce79e98b6cf0fcfd5ae02e/ambassador/ambassador/ir/ir.py#L583

Expected behavior
Some better error handling somewhere.

Versions (please complete the following information):

• Ambassador: 0.61.0
• Kubernetes environment BYO Kubernetes on AWS
• Version 1.14

Additional context
I did not see this with Ambassador v0.60.3.