test client: improve error handling
test client: improve error handling

Implement error handling as suggested in wth_connection documentation,
and add the final roundtrip too.

There is a commented-out call to trigger a protocol error. If you
uncomment that, the execution does not actually stop at receiving the
error because wth_connection does not implement the behaviour yet.

Signed-off-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

waltham/waltham,waltham/waltham