Feature/391 implement error interface

[psrebniak]

Fixes #391
Had to change apm.captureError behavior a little bit - instead of returning nil, Error with nil ErrorData is returned. Comments are adjusted.

I think Error.sent() name may be adjusted as there are more cases when ErrorData is nil.

Tests for causer and error interfaces added.

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[elasticcla]

Hi @psrebniak, we have found your signature in our records, but it seems like you have signed with a different e-mail than the one used in yout Git commit. Can you please add both of these e-mails into your Github profile (they can be hidden), so we can match your e-mails to your Github profile?