Large build results will not be returned from an agent successfully

When the agent returns a build result to the master, it places the whole result in an XMLRPC object. This can cause an OOME if it is large enough (e.g. in MB).

While Continuum should be doing more to incrementally fetch the build result, for now such a large result should produce a meaningful error that the master can display (since at the moment it doesn't provide any details in the log output window).