If can't find an expected piece of metadata, log and continue

On certain instance types (e.g. hi.4xlarge), AWS returns 404 for certain pieces of metadata (e.g. on a hi.4xlarge, metrics/vhostmd returns 404). A half-completed ec2 section in ohai is still more helpful than an empty ec2 section, so log the error and continue, rather than raising an exception.

See also: https://tickets.opscode.com/browse/OHAI-541