Fix findbugs warnings

[kushti]

Findbugs tool (running via "sbt findbugs") is finding a couple of high-priority warnings and a lot of medium-priority warnings. We need to fix them (most of claims are definitely reasonable). Good task for an external contributor!

[atapin]

@kushti I'd like to claim on this one

[kushti]

@atapin awesome, thanks in advance!

[atapin]

@kushti I took a glance at findbugs and it looked like java-ish to me. I'd suggest to consider https://github.com/wartremover/wartremover or https://github.com/sksamuel/scapegoat.