use more specific error messages in ec2 keystone auth

Currently in the ec2 keystone auth, when status code != 200 the
response.reason is used for the error message only if
status code == 401. This change uses response.reason for the error
message for any status code != 200.

It also changes a message "failure communicating with keystone"
to "failure parsing response from keystone."

Change-Id: Ifdfc30ff260298637c437e5758983db53a86c0dc
Merge "use more specific error messages in ec2 keystone auth"