HMP: Prepend errors with 'Error:'

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 <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Message-Id: <20190212134758[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>