fix: do not fail stop node if failed start node

[alanshaw]

If failed start node in before, do not fail stop node in after.

It reduces the noise from IPFS node start failures by fixing the after hook so it doesn't also error when trying to stop a node that didn't start.

e.g.

  4) .refs "before all" hook:
     Uncaught AssertionError: expected [Error: connect ECONNREFUSED 127.0.0.1:53477] to not exist
      at ipfsd.api.id (test\ping.spec.js:47:30)
      at send (node_modules\ipfsd-ctl\node_modules\ipfs-api\src\id.js:19:16)
      at f (node_modules\once\once.js:25:25)
      at ClientRequest.req.on (node_modules\ipfsd-ctl\node_modules\ipfs-api\src\utils\send-request.js:168:5)
      at Socket.socketErrorListener (_http_client.js:387:9)
      at emitErrorNT (internal/streams/destroy.js:64:8)
      at _combinedTickCallback (internal/process/next_tick.js:138:11)
      at process._tickCallback (internal/process/next_tick.js:180:9)

This PR stops us from also receiving this noise:

  5) .refs "after all" hook:
     TypeError: Cannot read property 'stop' of undefined
      at Context.after (test\refs.spec.js:52:25)

[daviddias]

LGTM