t: gateway-place-assign: check response.code first
t: gateway-place-assign: check response.code first

Before parsing the received data structure, the response status
should be checked to report the reason of a potential failure.
When that status is non-200 then it is possible that the data
is invalid (or empty) and thus we get incomprehensible error
output.

Fixes: 5a9ec64283c8 ("t: ui: test gateway endpoint")
Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server