Handle URIs that fail to parse

https://github.com/cloudfoundry/cloud_controller_ng/pull/159 exposed "bad" urls sent to the DEA
We saw the below in our logs

{"timestamp":1391435749.309297,"message":"staging.task.failed","log_level":"info","source":"Staging","data":{"app_guid":"13b010dc-8d26-4629-bc86-9ad09b87a88d","task_id":"05dece98d6984a31b67e3828be2ece5f","error":"bad argument (expected URI object or URI string)","backtrace":["/var/vcap/packages/ruby/lib/ruby/1.9.1/uri/common.rb:996:in `URI'","/var/vcap/packages/dea_next/lib/dea/staging/staging_message.rb:46:in `block in admin_buildpacks'","/var/vcap/packages/dea_next/lib/dea/staging/staging_message.rb:45:in `map'","/var/vcap/packages/dea_next/lib/dea/staging/staging_message.rb:45:in `admin_buildpacks'","/var/vcap/packages/dea_next/lib/dea/staging/staging_task_workspace.rb:18:in `initialize'","/var/vcap/packages/dea_next/lib/dea/staging/staging_task.rb:82:in `new'","/var/vcap/packages/dea_next/lib/dea/staging/staging_task.rb:82:in `workspace'","/var/vcap/packages/dea_next/lib/dea/staging/staging_task.rb:94:in `streaming_log_url'","/var/vcap/packages/dea_next/lib/dea/responders/staging.rb:119:in `block in notify_setup_completion'","/var/vcap/packages/dea_next/lib/dea/staging/staging_task.rb:142:in `call'","/var/vcap/packages/dea_next/lib/dea/staging/staging_task.rb:142:in `trigger_after_setup'","/var/vcap/packages/dea_next/lib/dea/staging/staging_task.rb:518:in `rescue in resolve_staging_setup'","/var/vcap/packages/dea_next/lib/dea/staging/staging_task.rb:497:in `resolve_staging_setup'","/var/vcap/packages/dea_next/lib/dea/staging/staging_task.rb:45:in `block in start'","/var/vcap/packages/dea_next/lib/dea/promise.rb:80:in `call'","/var/vcap/packages/dea_next/lib/dea/promise.rb:80:in `block in run'"]},"thread_id":8960140,"fiber_id":25176740,"process_id":15799,"file":"/var/vcap/packages/dea_next/lib/dea/staging/staging_task.rb","lineno":53,"method":"block in start"}