Catch errors and return something usefull

[bittersweet]

While logging activity Prooflink threw a 503 error, we should check the response of requests before handling them so we can provide useful errors to the users

Stacktrace from errrbit:

json-1.5.4/lib/json/common.rb → parse
json-1.5.4/lib/json/common.rb → parse
multi_json-1.3.6/lib/multi_json/adapters/json_common.rb → load
multi_json-1.3.6/lib/multi_json.rb → decode
prooflink_connect-0.0.23/lib/prooflink_connect/activity.rb → log