Set default http-based exception as `HttpError`

Since exceptions from oslo is used, ClientException doesn't have any
attributes related to HTTP, so `HttpError` should be set as default
http-based exception.

Also, attribute `code` is used in several project, so it should be
returned and marked as deprecated.

Partial-Bug: #1322183
Change-Id: I3c71e2d25d6e36b5bac0f2b3add74d6747cf7c25
Merge "Set default http-based exception as `HttpError`"