net: Improve -net nic error reporting

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

    $ qemu-system-x86_64 -net nic,netdev=nonexistent
    qemu-system-x86_64: -net nic,netdev=nonexistent: netdev 'nonexistent' not found
    qemu-system-x86_64: -net nic,netdev=nonexistent: Device 'nic' could not be initialized

Convert net_init_nic() to Error to get rid of the unwanted second
error message.

While there, tidy up an Overcapitalized Error Message.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1431691143-1015-4-git-send-email-armbru@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
