Need better error message when nova fails to connect to memcache

[Philip Choi]

Was trying to troubleshoot a nova-consoleauth token authentication issue and found that it didn't provide enough information to pinpoint a memcache connection issue. I think it will benefit everyone if we can slightly improve the cache_utils.py code if we can add some verbosity there, for example like stating connection failure to memcache or key looksup failure.

Here is an example of nova-consoleauth.log, it just mentioned about token checks failure but didn't mention any potential cause about it.
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2017-09-08 01:55:50.789 45899 INFO nova.consoleauth.manager [req-920d262b-6008-401c-bf4f-97f2a4d6e8df 4c5cfa26e24440dab733fa62d35e510d d8e10a9adc8c430aae6b1465eed37c10 - - -] Received Token: 59b8e706-f93c-437e-8a9a-1f00f2be0b17, {'instance_uuid': u'7d0e17e1-262f-4791-a0cd-bc593830f383', 'access_url': u'https://10.207.255.30:6082/spice_auto.html?token=59b8e706-f93c-437e-8a9a-1f00f2be0b17', 'token': u'59b8e706-f93c-437e-8a9a-1f00f2be0b17', 'last_activity_at': 1504835750.693094, 'internal_access_path': None, 'console_type': u'spice-html5', 'host': u'10.207.255.21', 'port': u'5901'}
2017-09-08 01:56:01.647 45899 INFO nova.consoleauth.manager [req-87741322-646d-4b1c-8b8e-f68c1e1ad89e - - - - -] Checking Token: 59b8e706-f93c-437e-8a9a-1f00f2be0b17, False
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[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/501928

[Philip Choi (phchoi)]

https://review.openstack.org/#/c/501928/ Proposing something like this, will this help?

[OpenStack Infra (hudson-openstack)]

Change abandoned by Stephen Finucane (<email address hidden>) on branch: master
Review: https://review.opendev.org/501928