Fail EC2 inventory better with restricted EC2 users

If a user has full EC2 access but no RDS access, the user
should still be able to query the EC2 inventory, although
will not obtain any information about any RDS instances.
Merge pull request #5236 from willthames/ec2_inv_rds_fail_safe

ec2 inventory: Fail EC2 inventory better with restricted EC2 users