HMP: Prepend errors with 'Error:'

Always make error messages start with 'Error:' as a fallback
to make sure that anything parsing them can tell it failed.

Note: Some places don't use hmp_handle_error

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20190212134758.10514-3-dgilbert@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>