Distributed build may never return

Under the circumstances outlined in CONTINUUM-2655, the build result is never returned to the server. In this case, the server never times out waiting for the response, and further pings do not encourage the agent to present the failure.

The build will run forever if it was forced and not respecting the timeout value. It can no longer be cancelled as it doesn't appear in the queue.

Two things are needed:

• A failure to respond should generate a new error response (with a simpler message less likely to fail)
• The server should check for a lack of response and end the build if the job has finished