Don't raise unnecessary stack traces in EC2 API

exception.EC2APIError exceptions are always expected, so no reason to have a
LOG.exception when one is raised.  Furthermore nova.api.ec2.ec2_error already
logs the exception so the duplicate log statement is just removed

Fix Bug 1180973

Change-Id: I4b42a9a764f6e15bb94a383e8a725d52d7436bd7
Merge "Don't raise unnecessary stack traces in EC2 API"