Misleading error when missing isolator plugin file

[ozdanborne]

This bug is on the new isolator module (JSON): If the Isolator calls the plugin but the plugin isn't in the expected location, it just reports an empty response.

slave_2       | I0904 23:44:57.972000   180 containerizer.cpp:614] Starting container 'b42ce056-ae79-453b-9dd1-6272264e01cb' for executor 'sample-flask-app.f359adbb-535e-11e5-89f6-0242ac11000c' of framework '20150904-233807-184553900-5050-1-0000'
slave_2       | I0904 23:44:58.120246   181 network_isolator.cpp:129] Got response from /isolator/build/../calico_isolator: 
slave_2       | I0904 23:44:58.120398   181 network_isolator.cpp:200] NetworkIsolator::prepare for container: b42ce056-ae79-453b-9dd1-6272264e01cb
slave_2       | E0904 23:44:58.120476   180 slave.cpp:3301] Container 'cc6c7cc6-c4ca-4910-bfb0-b1955f3fab84' for executor 'sample-flask-app-2.f359fbdc-535e-11e5-89f6-0242ac11000c' of framework '20150904-233807-184553900-5050-1-0000' failed to start: Error allocating IP from IPAM: Error parsing output '' to JSON stringsyntax error at line 1 near: 

Might be worthwhile to improve the logging to report that the file isn't there

[ozdanborne]

@karya0

[unclejack]

Hasn't this been fixed by the IPAM and isolator detection code? It shouldn't be a problem any more because net-modules won't activate if the isolator/ipam is missing.