client: pass up error message for unknown error

[adamvy]

[tulir]

Unknown errors intentionally exclude the error as it can be absolutely anything. Some other bridges put it in info -> go_error in case it's needed for metrics/logs/etc, but message is meant for things that are shown to users directly

[tulir]

(the error is already logged above the if)

[adamvy]

I guess, UNKNOWN_ERRORs should be minimized anyways, I kind of would prefer an unhelpful error message over nothing but i've put it in go_error fo rnow