WaitExecution will sometimes set the error on the rsp object, which we
were previously failing to check for

Because the default value of an int is 0, if the execute response result
was unset, we would sometimes incorrectly return the default value which
looks like a success code

Fixes #1 from this thread:
https://buildbuddy-corp.slack.com/archives/C07GMM2VBLY/p1734595606837269