Make plugin and engine warnings available to the user of the lib
Make plugin and engine warnings available to the user of the lib

The object returned from runner() now has a warnings property
which provides information about plugins and engines that could
not be loaded.

The decision of what to do with the information is up to the client
code (exit the process, throw an error etc).

shoreditch-ops/artillery,erikerikson/artillery-core,hassy/artillery,shoreditch-ops/artillery-core,hassy/artillery-core,shoreditch-ops/artillery,hassy/artillery,erikerikson/artillery-core,shoreditch-ops/artillery-core,hassy/artillery,hassy/artillery-core,shoreditch-ops/artillery