Lower log level of errors caused by user requests to INFO

Lower log level of errors caused by user requests to INFO

When webob exception is returned, error level log is
always recorded even though we return 4xx (client error)
to users. 4xx error in response code means errors due to
user requests, so error level log is not appropriate.
This commit lowers the log level to INFO for such events.

Currently only case I see is webob.exc.HTTPNotFound returned
by the policy engine when a user requests an operation
prohibited by the policy, but I think there is no reason
we deal with 404 specially in neutron.api.v2.resource.

Change-Id: I9f042a90c9bf528be7cb835d7fe818ed1879054b
Closes-Bug: #1473556