tap: Improve -netdev/netdev_add/-net/... bridge error reporting

When -netdev bridge fails, it first reports a specific error, then a
generic one, like this:

    $ qemu-system-x86_64 -netdev bridge,id=foo
    failed to launch bridge helper
    qemu-system-x86_64: -netdev bridge,id=foo: Device 'bridge' could not be initialized

The first message goes to stderr.  Wrong for HMP, because errors need
to go to the monitor there.

The second message goes to stderr for -netdev, to the monitor for HMP
netdev_add, and becomes the error reply for QMP netdev_add.

Convert net_bridge_run_helper() to Error, and propagate its errors
through net_init_bridge().  This ensures the error gets reported where
the user is, and suppresses the unwanted second message.

While there, improve the error messages a bit.

The above example becomes:

    $ qemu-system-x86_64 -netdev bridge,id=foo
    qemu-system-x86_64: -netdev bridge,id=foo: bridge helper failed

net_init_tap() also uses net_bridge_run_helper().  Propagate its
errors there as well.  Improves reporting these errors with -netdev
tap & friends.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1431691143-1015-7-git-send-email-armbru@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
