Return error details to users in "dns-create-private-domain"

Fixes bug 1135539
When SysLogHandler is configured in UDP socket mode, the user
who doesn't have the admin privilege got the unexpecting internal
server error(HTTP 500) in "nova dns-create-private-domain" command.
This bug occurs as the call graph drew below:

|--ProxyCallback._process_data():amqp.py
                          |--LOG.exception()
                          |    :
                          |    |--BaseHub.switch():hub.py
                          |                 |-- clear_sys_exc_info()
                          |
                          |--ctxt.reply(None, sys.exc_info(),..)
                                       |
                                       :
                                       |
                         |--serialize_remote_exception():common.py
                                       |--failure_info#(None,None,None)

The sys.exc_info() is cleared by the clear_sys_exc_info(), but it is
called again by the ctxt.reply().
This patch prevents the internal error and returns the details of
the forbidden error(HTTP 403) which is expected.

Change-Id: I76d4a938e66bfaf44423f7586e00db2caf9f5dde
Merge "Return error details to users in "dns-create-private-domain""